The eradication or abolition of suffering is the concept of using biotechnology to create a permanent absence of involuntary pain and suffering in all sentient beings.
Biology and medicine
The discovery of modern anesthesia
in the 19th century was an early breakthrough in the elimination of
pain during surgery, but acceptance was not universal. Some medical
practitioners at the time believed that anesthesia was an artificial and harmful intervention in the body's natural response to injury.
Opposition to anesthesia has since dissipated; however, the prospect of
eradicating pain raises similar concerns about interfering with life's
natural functions.
People who are naturally incapable of feeling pain or unpleasant sensations due to rare conditions like pain asymbolia or congenital insensitivity to pain have been studied to discover the biological and genetic reasons for their pain-free lives. A Scottish woman with a previously unreported genetic mutation in a FAAHpseudogene (dubbed FAAH-OUT) with resultant elevated anandamide
levels was reported in 2019 to be immune to anxiety, unable to
experience fear, and insensitive to pain. The frequent burns and cuts
she had due to her full hypoalgesia healed quicker than average.
In 1990, Medical Hypotheses published an article by L. S. Mancini on the "genetic engineering of a world without pain":
A hypothesis is presented to the
effect that everything adaptive which is achievable with a mind capable
of experiencing varying degrees of both pleasure and pain (the human condition as we know it) could be achieved with a mind capable of experiencing only varying degrees of pleasure.
The development of gene editing techniques like CRISPR has raised the prospect that "scientists can identify the causes of certain unusual people's physical superpowers and use gene editing to grant them to others." Geneticist George Church has commented on the potential future of replacing pain with a painless sensory system:
I imagine what this would be like
on another planet and in the future, and... given that imagined future,
whether we would be willing to come back to where we are now. Rather than saying whether we're willing to go forward... ask whether you're willing to come back.
Ethics and philosophy
Ethicists and philosophers in the schools of hedonism and utilitarianism, especially negative utilitarianism, have debated the merits of eradicating suffering. Transhumanist philosopher David Pearce, in The Hedonistic Imperative (1995), argues that the abolition of suffering is both technically feasible and an issue of moral urgency, stating that: "It is predicted that the world's last unpleasant experience will be a precisely dateable event."
The philosopher Nick Bostrom, director of the Future of Humanity Institute,
advises a more cautious approach due to pain's function in protecting
individuals from harm. However, Bostrom supports the core idea of using
biotechnology to get rid of "a huge amount of unnecessary and undeserved
suffering."
It has also been argued that the eradication of suffering through
biotechnology may bring about unwanted consequences, and arguments have
been made that transhumanism is not the only philosophy worthy of
consideration regarding the question of suffering — many people view
suffering as one aspect in a dualist understanding of psychological and physical functioning, without which pleasure could not exist.
Animal welfare
In 2009, Adam Shriver suggested replacing animals in factory farming with genetically engineered animals with a reduced or absent capacity to suffer and feel pain. Shriver and McConnachie argued that people who wish to improve animal welfare should support gene editing in addition to plant-based diets and cultured meat.
Katrien Devolder and Matthias Eggel proposed gene editing research animals
to remove pain and suffering. This would be an intermediate step
towards eventually stopping all experimentation on animals and adopting alternatives.
Concerning wild-animal suffering, CRISPR-based gene drives have been suggested as a cost-effective way of spreading benign alleles in sexually reproducing species. To limit gene drives spreading indefinitely (for test programmes, for example), the Sculpting Evolution group at the MIT Media Lab developed a self-exhausting form of CRISPR-based gene drive called a "daisy-chain drive." For potential adverse effects of a gene drive, "[s]everal genetic
mechanisms for limiting or eliminating gene drives have been proposed
and/or developed, including synthetic resistance, reversal drives, and
immunizing reversal drives."
Ernst Haeckel (1834–1919) attempted to synthesize the ideas of Lamarckism and Goethe's Naturphilosophie with Charles Darwin's
concepts. While often seen as rejecting Darwin's theory of branching
evolution for a more linear Lamarckian view of progressive evolution,
this is not accurate: Haeckel used the Lamarckian picture to describe
the ontogenetic and phylogenetic history of individual species, but
agreed with Darwin about the branching of all species from one, or a
few, original ancestors. Since early in the twentieth century, Haeckel's "biogenetic law" has been refuted on many fronts.
Haeckel formulated his theory as "Ontogeny recapitulates
phylogeny". The notion later became simply known as the recapitulation
theory. Ontogeny is the growth (size change) and development (structure change) of an individual organism; phylogeny is the evolutionary
history of a species. Haeckel claimed that the development of advanced
species passes through stages represented by adult organisms of more
primitive species.
Otherwise put, each successive stage in the development of an
individual represents one of the adult forms that appeared in its
evolutionary history.
For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches
in the neck of the human embryo not only roughly resembled gill slits
of fish, but directly represented an adult "fishlike" developmental
stage, signifying a fishlike ancestor. Embryonic pharyngeal slits, which
form in many animals when the thin branchial plates separating
pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes.
Haeckel produced several embryo drawings
that often overemphasized similarities between embryos of related
species. Modern biology rejects the literal and universal form of
Haeckel's theory, such as its possible application to behavioural
ontogeny, i.e. the psychomotor development of young animals and human
children.
Contemporary criticism
Drawing by Wilhelm His of chick brain compared to folded rubber tube, 1874. Ag (Anlage) = Optic lobes, matching bulges in rubber tube.
Haeckel's theory and drawings were criticised by his contemporary, the anatomist Wilhelm His Sr. (1831–1904), who had developed a rival "causal-mechanical theory" of human embryonic development.His's work specifically criticised Haeckel's methodology, arguing that
the shapes of embryos were caused most immediately by mechanical
pressures resulting from local differences in growth. These differences
were, in turn, caused by "heredity". He compared the shapes of embryonic
structures to those of rubber tubes that could be slit and bent,
illustrating these comparisons with accurate drawings. Stephen Jay Gould noted in his 1977 book Ontogeny and Phylogeny
that His's attack on Haeckel's recapitulation theory was far more
fundamental than that of any empirical critic, as it effectively stated
that Haeckel's "biogenetic law" was irrelevant.
Darwin proposed that embryos resembled each other since they shared a
common ancestor, which presumably had a similar embryo, but that
development did not necessarily recapitulate phylogeny: he saw no reason
to suppose that an embryo at any stage resembled an adult of any
ancestor. Darwin supposed further that embryos were subject to less
intense selection pressure than adults, and had therefore changed less.
Modern status
Modern evolutionary developmental biology
(evo-devo) follows von Baer, rather than Darwin, in pointing to active
evolution of embryonic development as a significant means of changing
the morphology of adult bodies. Two of the key principles of evo-devo, namely that changes in the timing (heterochrony) and positioning (heterotopy)
within the body of aspects of embryonic development would change the
shape of a descendant's body compared to an ancestor's, were first
formulated by Haeckel in the 1870s. These elements of his thinking about
development have thus survived, whereas his theory of recapitulation
has not.
The Haeckelian form of recapitulation theory is considered defunct. Embryos do undergo a period or phylotypic stage where their morphology is strongly shaped by their phylogenetic position,
rather than selective pressures, but that means only that they resemble
other embryos at that stage, not ancestral adults as Haeckel had
claimed. The modern view is summarised by the University of California Museum of Paleontology:
Embryos do reflect the course of
evolution, but that course is far more intricate and quirky than Haeckel
claimed. Different parts of the same embryo can even evolve in
different directions. As a result, the Biogenetic Law was abandoned, and
its fall freed scientists to appreciate the full range of embryonic
changes that evolution can produce—an appreciation that has yielded
spectacular results in recent years as scientists have discovered some
of the specific genes that control development.
Applications to other areas
The idea that ontogeny recapitulates phylogeny has been applied to some other areas.
Cognitive development
English philosopher Herbert Spencer
was one of the most energetic proponents of evolutionary ideas to
explain many phenomena. In 1861, five years before Haeckel first
published on the subject, Spencer proposed a possible basis for a
cultural recapitulation theory of education with the following claim:
If
there be an order in which the human race has mastered its various
kinds of knowledge, there will arise in every child an aptitude to
acquire these kinds of knowledge in the same order... Education is a
repetition of civilization in little.
— Herbert Spencer
G. Stanley Hall
used Haeckel's theories as the basis for his theories of child
development. His most influential work, "Adolescence: Its Psychology and
Its Relations to Physiology, Anthropology, Sociology, Sex, Crime,
Religion and Education" in 1904
suggested that each individual's life course recapitulated humanity's
evolution from "savagery" to "civilization". Though he has influenced
later childhood development theories, Hall's conception is now generally
considered racist.
Developmental psychologist Jean Piaget favored a weaker version of the formula, according to which ontogeny parallels phylogeny because the two are subject to similar external constraints.
The Austrian pioneer of psychoanalysis, Sigmund Freud,
also favored Haeckel's doctrine. He was trained as a biologist under
the influence of recapitulation theory during its heyday, and retained a
Lamarckian outlook with justification from the recapitulation theory.
Freud also distinguished between physical and mental recapitulation,
in which the differences would become an essential argument for his theory of neuroses.
In the late 20th century, studies of symbolism and learning in
the field of cultural anthropology suggested that "both biological
evolution and the stages in the child's cognitive development follow
much the same progression of evolutionary stages as that suggested in
the archaeological record".
Music criticism
The musicologist Richard Taruskin
in 2005 applied the phrase "ontogeny becomes phylogeny" to the process
of creating and recasting music history, often to assert a perspective
or argument. For example, the peculiar development of the works by
modernist composer Arnold Schoenberg
(here an "ontogeny") is generalized in many histories into a
"phylogeny" – a historical development ("evolution") of Western music
toward atonal styles of which Schoenberg is a representative. Such historiographies
of the "collapse of traditional tonality" are faulted by music
historians as asserting a rhetorical rather than historical point about
tonality's "collapse".
Taruskin also developed a variation of the motto into the pun "ontogeny recapitulates ontology" to refute the concept of "absolute music" advancing the socio-artistic theories of the musicologist Carl Dahlhaus. Ontology
is the investigation of what exactly something is, and Taruskin asserts
that an art object becomes that which society and succeeding
generations made of it. For example, Johann Sebastian Bach's St. John Passion, composed in the 1720s, was appropriated by the Nazi regime in the 1930s for propaganda. Taruskin claims the historical development of the St John Passion (its ontogeny) as a work with an anti-Semitic
message does, in fact, inform the work's identity (its ontology), even
though that was an unlikely concern of the composer. Music or even an
abstract visual artwork can not be truly autonomous ("absolute") because
it is defined by its historical and social reception.
Analysis of lunar rocks
published in a 2016 report suggests that the impact might have been a
direct hit, causing a fragmentation and thorough mixing of both parent
bodies. The giant-impact hypothesis is currently the favored hypothesis for lunar formation among astronomers. Evidence that supports this hypothesis include:
The stable isotope ratios of lunar and terrestrial rock are identical, implying a common origin.
The Earth–Moon system contains an anomalously high angular momentum,
meaning the momentum contained in Earth's rotation, the Moon's rotation
and the Moon revolving around Earth is significantly higher than the
other terrestrial planets. A giant impact might have supplied this excess momentum.
Moon samples indicate that the Moon was once molten to a substantial, but unknown, depth. This might have required much more energy than predicted to be available from the accretion of a celestial body of the Moon's size and mass. An extremely energetic process, such as a giant impact, could provide this energy.
The Moon has a relatively small ironcore, which gives it a lower density
than Earth. Computer models of a giant impact of a Mars-sized body with
Earth indicate the impactor's core would likely penetrate deep into
Earth and fuse with its own core. This would leave the Moon, which was
formed from the ejecta of lighter crust and mantle fragments that went beyond the Roche limit and were not pulled back by gravity to re-fuse with proto-Earth, with less remaining metallic iron than other planetary bodies.
The Moon is depleted in volatile
elements compared to Earth. Vaporizing at comparably lower
temperatures, they could be lost in a high-energy event, with the Moon's
smaller gravity unable to recapture them while Earth did.
However, there remain several questions concerning the best current models of the giant-impact hypothesis. The energy of such a giant impact is predicted to have heated Earth to produce a global magma ocean, and evidence of the resultant planetary differentiation of the heavier material sinking into Earth's mantle has been documented.
However, there is no self-consistent model that starts with the
giant-impact event and follows the evolution of the debris into a single
moon. Other remaining questions include when the Moon lost its share of
volatile elements and why Venus – which experienced giant impacts during its formation – does not host a similar moon.
History
In 1898, George Darwin
made the suggestion that Earth and the Moon were once a single body.
Darwin's hypothesis was that a molten Moon had been spun from Earth
because of centrifugal forces, and this became the dominant academic explanation. Using Newtonian mechanics,
he calculated that the Moon had orbited much more closely in the past
and was drifting away from Earth. This drifting was later confirmed by American and Soviet experiments, using laser ranging targets placed on the Moon.
Nonetheless, Darwin's calculations could not resolve the
mechanics required to trace the Moon back to the surface of Earth. In
1946, Reginald Aldworth Daly of Harvard University
challenged Darwin's explanation, adjusting it to postulate that the
creation of the Moon was caused by an impact rather than centrifugal
forces.
Little attention was paid to Professor Daly's challenge until a
conference on satellites in 1974, during which the idea was reintroduced
and later published and discussed in Icarus in 1975 by William K. Hartmann and Donald R. Davis.
Their models suggested that, at the end of the planet formation period,
several satellite-sized bodies had formed that could collide with the
planets or be captured. They proposed that one of these objects might
have collided with Earth, ejecting refractory, volatile-poor dust that
could coalesce to form the Moon. This collision could potentially
explain the unique geological and geochemical properties of the Moon.
A similar approach was taken by Canadian astronomer Alastair G. W. Cameron and American astronomer William R. Ward, who suggested that the Moon was formed by the tangential
impact upon Earth of a body the size of Mars. It is hypothesized that
most of the outer silicates of the colliding body would be vaporized,
whereas a metallic core would not. Hence, most of the collisional
material sent into orbit would consist of silicates, leaving the
coalescing Moon deficient in iron. The more volatile materials that were
emitted during the collision probably would escape the Solar System,
whereas silicates would tend to coalesce.
Eighteen months prior to an October 1969 conference on lunar
origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged
fellow lunar scientists: "You have eighteen months. Go back to your
Apollo data, go back to your computer, and do whatever you have to, but
make up your mind. Don't come to our conference unless you have
something to say about the Moon's birth." At the 1969 conference at Kona, Hawaii, the giant-impact hypothesis emerged as the most favored hypothesis.
Before the conference, there were
partisans of the three "traditional" theories, plus a few people who
were starting to take the giant impact seriously, and there was a huge
apathetic middle who didn't think the debate would ever be resolved.
Afterward, there were essentially only two groups: the giant impact camp
and the agnostics.
The name of the hypothesised protoplanet is derived from the mythical GreektitanTheia/ˈθiːə/, who gave birth to the Moon goddess Selene. This designation was proposed initially by the English geochemist Alex N. Halliday in 2000 and has become accepted in the scientific community. According to modern theories of planet formation, Theia was part of a
population of Mars-sized bodies that existed in the Solar System 4.5
billion years ago. One of the attractive features of the giant-impact
hypothesis is that the formation of the Moon and Earth align; during the
course of its formation, Earth is thought to have experienced dozens of
collisions with planet-sized bodies. The Moon-forming collision would
have been only one such "giant impact" but certainly the last
significant impactor event. The Late Heavy Bombardment by much smaller asteroids may have occurred later – approximately 3.9 billion years ago.
Basic model
Simplistic representation of the giant-impact hypothesis.
Astronomers think the collision between Earth and Theia happened at about 4.4 to 4.45 bya; about 0.1 billion years after the Solar System began to form. In astronomical terms, the impact would have been of moderate velocity. Theia is thought to have struck Earth at an oblique angle
when Earth was nearly fully formed. Computer simulations of this
"late-impact" scenario suggest an initial impactor velocity below 4
kilometres per second (2.5 mi/s) at "infinity" (far enough that
gravitational attraction is not a factor), increasing as it approached
to over 9.3 km/s (5.8 mi/s) at impact, and an impact angle of about 45°. However, oxygenisotope abundance in lunar rock suggests "vigorous mixing" of Theia and Earth, indicating a steep impact angle. Theia's iron core would have sunk into the young Earth's core, and most of Theia's mantle accreted onto Earth's mantle. However, a significant portion of the mantle material from both Theia and Earth would have been ejected into orbit around Earth (if ejected with velocities between orbital velocity and escape velocity) or into individual orbits around the Sun (if ejected at higher velocities).
Modelling
has hypothesised that material in orbit around Earth may have accreted
to form the Moon in three consecutive phases; accreting first from the
bodies initially present outside Earth's Roche limit,
which acted to confine the inner disk material within the Roche limit.
The inner disk slowly and viscously spread back out to Earth's Roche
limit, pushing along outer bodies via resonant interactions. After
several tens of years, the disk spread beyond the Roche limit, and
started producing new objects that continued the growth of the Moon,
until the inner disk was depleted in mass after several hundreds of
years. Material in stable Kepler orbits was thus likely to hit the Earth–Moon system sometime later (because the Earth–Moon system's Kepler orbit around the Sun also remains stable). Estimates based on computer simulations
of such an event suggest that some twenty percent of the original mass
of Theia would have ended up as an orbiting ring of debris around Earth,
and about half of this matter coalesced into the Moon. Earth would have
gained significant amounts of angular momentum and mass
from such a collision. Regardless of the speed and tilt of Earth's
rotation before the impact, it would have experienced a day some five
hours long after the impact, and Earth's equator and the Moon's orbit
would have become coplanar.
Not all of the ring material need have been swept up right away:
the thickened crust of the Moon's far side suggests the possibility that
a second moon about 1,000 km (620 mi) in diameter formed in a Lagrange point
of the Moon. The smaller moon may have remained in orbit for tens of
millions of years. As the two moons migrated outward from Earth, solar
tidal effects would have made the Lagrange orbit unstable, resulting in a
slow-velocity collision that "pancaked" the smaller moon onto what is
now the far side of the Moon, adding material to its crust.
Lunar magma cannot pierce through the thick crust of the far side, causing fewer lunar maria, while the near side has a thin crust displaying the large maria visible from Earth.
Above
a high resolution threshold for simulations, a study published in 2022
finds that giant impacts can immediately place a satellite with similar
mass and iron content to the Moon into orbit far outside Earth's Roche
limit. Even satellites that initially pass within the Roche limit can
reliably and predictably survive, by being partially stripped and then
torqued onto wider, stable orbits. Furthermore, the outer layers of
these directly formed satellites are molten over cooler interiors and
are composed of around 60% proto-Earth material. This could alleviate
the tension between the Moon's Earth-like isotopic composition and the
different signature expected for the impactor. Immediate formation opens
up new options for the Moon's early orbit and evolution, including the
possibility of a highly tilted orbit to explain the lunar inclination,
and offers a simpler, single-stage scenario for the origin of the Moon.
Composition
In 2001, a team at the Carnegie Institution of Washington reported that the rocks from the Apollo program carried an isotopic signature that was identical with rocks from Earth, and were different from almost all other bodies in the Solar System. In 2014, a team in Germany reported that the Apollo samples had a slightly different isotopic signature from Earth rocks. The difference was slight, but statistically significant. One possible explanation is that Theia formed near Earth.
This empirical data showing close similarity of composition can
be explained only by the standard giant-impact hypothesis, as it is
extremely unlikely that two bodies prior to collision had such similar
composition.
Equilibration hypothesis
In
2007, researchers from the California Institute of Technology showed
that the likelihood of Theia having an identical isotopic signature as
Earth was very small (less than 1 percent).
They proposed that in the aftermath of the giant impact, while Earth
and the proto-lunar disc were molten and vaporised, the two reservoirs
were connected by a common silicate vapor atmosphere and that the
Earth–Moon system became homogenised by convective stirring while the
system existed in the form of a continuous fluid. Such an
"equilibration" between the post-impact Earth and the proto-lunar disc
is the only proposed scenario that explains the isotopic similarities of
the Apollo rocks with rocks from Earth's interior. For this scenario to
be viable, however, the proto-lunar disc would have to endure for about
100 years. Work is ongoing to determine whether or not this is possible.
Direct collision hypothesis
According to research (2012) to explain similar compositions of the Earth and the Moon based on simulations at the University of Bern
by physicist Andreas Reufer and his colleagues, Theia collided directly
with Earth instead of barely swiping it. The collision speed may have
been higher than originally assumed, and this higher velocity may have
totally destroyed Theia. According to this modification, the composition
of Theia is not so restricted, making a composition of up to 50% water
ice possible.
Synestia hypothesis
One
effort, in 2018, to homogenise the products of the collision was to
energise the primary body by way of a greater pre-collision rotational
speed. This way, more material from the primary body would be spun off
to form the Moon. Further computer modelling determined that the
observed result could be obtained by having the pre-Earth body spinning
very rapidly, so much so that it formed a new celestial object which was
given the name 'synestia'.
This is an unstable state that could have been generated by yet
another collision to get the rotation spinning fast enough. Further
modelling of this transient structure has shown that the primary body
spinning as a doughnut-shaped object (the synestia) existed for about a
century (a very short time) before it cooled down and gave birth to Earth and the Moon.
Terrestrial magma ocean hypothesis
Another
model, in 2019, to explain the similarity of Earth and the Moon's
compositions posits that shortly after Earth formed, it was covered by a
sea of hot magma,
while the impacting object was likely made of solid material. Modelling
suggests that this would lead to the impact heating the magma much more
than solids from the impacting object, leading to more material being
ejected from the proto-Earth, so that about 80% of the Moon-forming
debris originated from the proto-Earth. Many prior models had suggested
80% of the Moon coming from the impactor.
Evidence
Indirect evidence for the giant impact scenario comes from rocks collected during the Apollo Moon landings, which show oxygenisotope ratios nearly identical to those of Earth. The highly anorthositic composition of the lunar crust, as well as the existence of KREEP-rich
samples, suggest that a large portion of the Moon once was molten; and a
giant impact scenario could easily have supplied the energy needed to
form such a magma ocean. Several lines of evidence show that if the Moon has an iron-rich
core, it must be a small one. In particular, the mean density, moment
of inertia, rotational signature, and magnetic induction response of the
Moon all suggest that the radius of its core is less than about 25% the
radius of the Moon, in contrast to about 50% for most of the other terrestrial
bodies. Appropriate impact conditions satisfying the angular momentum
constraints of the Earth–Moon system yield a Moon formed mostly from the
mantles of Earth and the impactor, while the core of the impactor accretes to Earth. Earth has the highest density of all the planets in the Solar System;
the absorption of the core of the impactor body explains this
observation, given the proposed properties of the early Earth and Theia.
Comparison of the zinc isotopic composition of lunar samples with that of Earth and Mars rocks provides further evidence for the impact hypothesis. Zinc is strongly fractionated when volatilised in planetary rocks, but not during normal igneous processes,
so zinc abundance and isotopic composition can distinguish the two
geological processes. Moon rocks contain more heavy isotopes of zinc,
and overall less zinc, than corresponding igneous Earth or Mars rocks,
which is consistent with zinc being depleted from the Moon through
evaporation, as expected for the giant impact origin.
Collisions between ejecta escaping Earth's gravity and asteroids
would have left impact heating signatures in stony meteorites; analysis
based on assuming the existence of this effect has been used to date the
impact event to 4.47 billion years ago, in agreement with the date
obtained by other means.
Warm silica-rich dust and abundant SiO gas, products of high
velocity impacts – over 10 km/s (6.2 mi/s) – between rocky bodies, have
been detected by the Spitzer Space Telescope around the nearby (29 pc distant) young (~12 My old) star HD 172555 in the Beta Pictoris moving group. A belt of warm dust in a zone between 0.25AU and 2AU from the young star HD 23514 in the Pleiades
cluster appears similar to the predicted results of Theia's collision
with the embryonic Earth, and has been interpreted as the result of
planet-sized objects colliding with each other. A similar belt of warm dust was detected around the star BD+20°307 (HIP 8920, SAO 75016).
On 1 November 2023, scientists reported that, according to
computer simulations, remnants of Theia could be still visible inside
the Earth as two giant anomalies of the Earth's mantle.
Difficulties
This
lunar origin hypothesis has some difficulties that have yet to be
resolved. For example, the giant-impact hypothesis implies that a
surface magma ocean would have formed following the impact. Yet there is
no evidence that Earth ever had such a magma ocean and it is likely
there exists material that has never been processed in a magma ocean.
Composition
A number of compositional inconsistencies need to be addressed.
The ratios of the Moon's volatile elements are not explained by
the giant-impact hypothesis. If the giant-impact hypothesis is correct,
these ratios must be due to some other cause.
The presence of volatiles such as water trapped in lunar basalts and carbon emissions from the lunar surface is more difficult to explain if the Moon was caused by a high-temperature impact.
The iron oxide (FeO) content (13%) of the Moon, intermediate between
that of Mars (18%) and the terrestrial mantle (8%), rules out most of
the source of the proto-lunar material from Earth's mantle.
If the bulk of the proto-lunar material had come from an impactor, the Moon should be enriched in siderophilic elements, when, in fact, it is deficient in them.
The Moon's oxygen isotopic ratios are essentially identical to those of Earth. Oxygen isotopic ratios, which may be measured very precisely, yield a unique and distinct signature for each Solar System body. If a separate proto-planet Theia had existed, it probably would have had a different oxygen isotopic signature than Earth, as would the ejected mixed material.
The Moon's titanium isotope ratio (50Ti/47Ti)
appears so close to Earth's (within 4 ppm), that little if any of the
colliding body's mass could likely have been part of the Moon.
Lack of a Venusian moon
If
the Moon was formed by such an impact, it is possible that other inner
planets also may have been subjected to comparable impacts. A moon that
formed around Venus
by this process would have been unlikely to escape. If such a
moon-forming event had occurred there, a possible explanation of why the
planet does not have such a moon might be that a second collision
occurred that countered the angular momentum from the first impact.
Another possibility is that the strong tidal forces from the Sun would
tend to destabilise the orbits of moons around close-in planets. For
this reason, if Venus's slow rotation rate began early in its history,
any satellites larger than a few kilometers in diameter would likely
have spiraled inwards and collided with Venus.
Simulations of the chaotic period of terrestrial planet formation
suggest that impacts like those hypothesised to have formed the Moon
were common. For typical terrestrial planets with a mass of 0.5 to 1
Earth masses, such an impact typically results in a single moon
containing 4% of the host planet's mass. The inclination of the
resulting moon's orbit is random, but this tilt affects the subsequent
dynamic evolution of the system. For example, some orbits may cause the
moon to spiral back into the planet. Likewise, the proximity of the
planet to the star will also affect the orbital evolution. The net
effect is that it is more likely for impact-generated moons to survive
when they orbit more distant terrestrial planets and are aligned with
the planetary orbit.
Possible origin of Theia
One suggested pathway for the impact as viewed from the direction of Earth's south pole (not to scale).
In 2004, Princeton University mathematician Edward Belbruno and astrophysicist J. Richard Gott III proposed that Theia coalesced at the L4 or L5Lagrangian point relative to Earth (in about the same orbit and about 60° ahead or behind), similar to a trojan asteroid. Two-dimensional computer models suggest that the stability of Theia's proposed trojan orbit would have been affected when its growing mass exceeded a threshold of approximately 10% of Earth's mass (the mass of Mars). In this scenario, gravitational perturbations by planetesimals
caused Theia to depart from its stable Lagrangian location, and
subsequent interactions with proto-Earth led to a collision between the
two bodies.
In 2008, evidence was presented that suggests that the collision might have occurred later than the accepted value of 4.53 Gya, at approximately 4.48 Gya.
A 2014 comparison of computer simulations with elemental abundance
measurements in Earth's mantle indicated that the collision occurred
approximately 95 My after the formation of the Solar System.
It has been suggested that other significant objects might have
been created by the impact, which could have remained in orbit between
Earth and the Moon, stuck in Lagrangian points. Such objects might have
stayed within the Earth–Moon system for as long as 100 million years,
until the gravitational tugs of other planets destabilised the system
enough to free the objects.
A study published in 2011 suggested that a subsequent collision between
the Moon and one of these smaller bodies caused the notable differences
in physical characteristics between the two hemispheres of the Moon.
This collision, simulations have supported, would have been at a low
enough velocity so as not to form a crater; instead, the material from
the smaller body would have spread out across the Moon (in what would
become its far side), adding a thick layer of highlands crust. The resulting mass irregularities would subsequently produce a gravity gradient that resulted in tidal locking of the Moon so that today, only the near side remains visible from Earth. However, mapping by the GRAIL mission has ruled out this scenario.
In 2019, a team at the University of Münster
reported that the molybdenum isotopic composition in Earth's primitive
mantle originates from the outer Solar System, hinting at the source of
water on Earth. One possible explanation is that Theia originated in the
outer Solar System.
Other mechanisms that have been suggested at various times for the
Moon's origin are that the Moon was spun off from Earth's molten surface
by centrifugal force; that it was formed elsewhere and was subsequently captured by Earth's gravitational field; or that Earth and the Moon formed at the same time and place from the same accretion disk. None of these hypotheses can account for the high angular momentum of the Earth–Moon system.
Another hypothesis attributes the formation of the Moon to the
impact of a large asteroid with Earth much later than previously
thought, creating the satellite primarily from debris from Earth. In
this hypothesis, the formation of the Moon occurs 60–140 million years
after the formation of the Solar System (as compared to hypothesized
Theia impact at 4.527 ± 0.010 billion years).
The asteroid impact in this scenario would have created a magma ocean
on Earth and the proto-Moon with both bodies sharing a common plasma
metal vapor atmosphere. The shared metal vapor bridge would have allowed
material from Earth and the proto-Moon to exchange and equilibrate into
a more common composition.
Yet another hypothesis proposes that the Moon and Earth formed
together, not from the collision of once-distant bodies. This model,
published in 2012 by Robin M. Canup,
suggests that the Moon and Earth formed from a massive collision of two
planetary bodies, each larger than Mars, which then re-collided to form
what is now called Earth. After the re-collision, Earth was surrounded by a disk of material
which accreted to form the Moon. This hypothesis could explain evidence
that others do not.
The theory of recapitulation, also called the biogenetic law or embryological parallelism—often expressed using Ernst Haeckel's phrase "ontogeny recapitulates phylogeny"—is an historical hypothesis that the development of the embryo of an animal, from fertilization to gestation or hatching (ontogeny), goes through stages resembling or representing successive adult stages in the evolution of the animal's remote ancestors (phylogeny). It was formulated in the 1820s by Étienne Serres based on the work of Johann Friedrich Meckel, after whom it is also known as Meckel–Serres law.
Since embryos also evolve in different ways,
the shortcomings of the theory had been recognized by the early 20th
century, and it had been relegated to "biological mythology" by the mid-20th century.
Analogies to recapitulation theory have been formulated in other fields, including cognitive development and music criticism.
The embryological theory was formalised by Serres in 1824–1826,
based on Meckel's work, in what became known as the "Meckel-Serres Law".
This attempted to link comparative embryology with a "pattern of unification" in the organic world. It was supported by Étienne Geoffroy Saint-Hilaire,
and became a prominent part of his ideas. It suggested that past
transformations of life could have been through environmental causes
working on the embryo, rather than on the adult as in Lamarckism. These naturalistic ideas led to disagreements with Georges Cuvier. The theory was widely supported in the Edinburgh and London schools of higher anatomy around 1830, notably by Robert Edmond Grant, but was opposed by Karl Ernst von Baer's ideas of divergence, and attacked by Richard Owen in the 1830s.
Ernst Haeckel (1834–1919) attempted to synthesize the ideas of Lamarckism and Goethe's Naturphilosophie with Charles Darwin's
concepts. While often seen as rejecting Darwin's theory of branching
evolution for a more linear Lamarckian view of progressive evolution,
this is not accurate: Haeckel used the Lamarckian picture to describe
the ontogenetic and phylogenetic history of individual species, but
agreed with Darwin about the branching of all species from one, or a
few, original ancestors. Since early in the twentieth century, Haeckel's "biogenetic law" has been refuted on many fronts.
Haeckel formulated his theory as "Ontogeny recapitulates
phylogeny". The notion later became simply known as the recapitulation
theory. Ontogeny is the growth (size change) and development (structure change) of an individual organism; phylogeny is the evolutionary
history of a species. Haeckel claimed that the development of advanced
species passes through stages represented by adult organisms of more
primitive species.
Otherwise put, each successive stage in the development of an
individual represents one of the adult forms that appeared in its
evolutionary history.
For example, Haeckel proposed that the pharyngeal grooves between the pharyngeal arches
in the neck of the human embryo not only roughly resembled gill slits
of fish, but directly represented an adult "fishlike" developmental
stage, signifying a fishlike ancestor. Embryonic pharyngeal slits, which
form in many animals when the thin branchial plates separating
pharyngeal pouches and pharyngeal grooves perforate, open the pharynx to the outside. Pharyngeal arches appear in all tetrapod embryos: in mammals, the first pharyngeal arch develops into the lower jaw (Meckel's cartilage), the malleus and the stapes.
Haeckel produced several embryo drawings
that often overemphasized similarities between embryos of related
species. Modern biology rejects the literal and universal form of
Haeckel's theory, such as its possible application to behavioural
ontogeny, i.e. the psychomotor development of young animals and human
children.
Contemporary criticism
Drawing by Wilhelm His of chick brain compared to folded rubber tube, 1874. Ag (Anlage) = Optic lobes, matching bulges in rubber tube.
Haeckel's theory and drawings were criticised by his contemporary, the anatomist Wilhelm His Sr. (1831–1904), who had developed a rival "causal-mechanical theory" of human embryonic development. His's work specifically criticised Haeckel's methodology, arguing that
the shapes of embryos were caused most immediately by mechanical
pressures resulting from local differences in growth. These differences
were, in turn, caused by "heredity". He compared the shapes of embryonic
structures to those of rubber tubes that could be slit and bent,
illustrating these comparisons with accurate drawings. Stephen Jay Gould noted in his 1977 book Ontogeny and Phylogeny
that His's attack on Haeckel's recapitulation theory was far more
fundamental than that of any empirical critic, as it effectively stated
that Haeckel's "biogenetic law" was irrelevant.
Darwin proposed that embryos resembled each other since they shared a
common ancestor, which presumably had a similar embryo, but that
development did not necessarily recapitulate phylogeny: he saw no reason
to suppose that an embryo at any stage resembled an adult of any
ancestor. Darwin supposed further that embryos were subject to less
intense selection pressure than adults, and had therefore changed less.
Modern status
Modern evolutionary developmental biology
(evo-devo) follows von Baer, rather than Darwin, in pointing to active
evolution of embryonic development as a significant means of changing
the morphology of adult bodies. Two of the key principles of evo-devo, namely that changes in the timing (heterochrony) and positioning (heterotopy)
within the body of aspects of embryonic development would change the
shape of a descendant's body compared to an ancestor's, were first
formulated by Haeckel in the 1870s. These elements of his thinking about
development have thus survived, whereas his theory of recapitulation
has not.
The Haeckelian form of recapitulation theory is considered defunct. Embryos do undergo a period or phylotypic stage where their morphology is strongly shaped by their phylogenetic position,
rather than selective pressures, but that means only that they resemble
other embryos at that stage, not ancestral adults as Haeckel had
claimed. The modern view is summarised by the University of California Museum of Paleontology:
Embryos do reflect the course of
evolution, but that course is far more intricate and quirky than Haeckel
claimed. Different parts of the same embryo can even evolve in
different directions. As a result, the Biogenetic Law was abandoned, and
its fall freed scientists to appreciate the full range of embryonic
changes that evolution can produce—an appreciation that has yielded
spectacular results in recent years as scientists have discovered some
of the specific genes that control development.
Applications to other areas
The idea that ontogeny recapitulates phylogeny has been applied to some other areas.
Cognitive development
English philosopher Herbert Spencer
was one of the most energetic proponents of evolutionary ideas to
explain many phenomena. In 1861, five years before Haeckel first
published on the subject, Spencer proposed a possible basis for a
cultural recapitulation theory of education with the following claim:
If
there be an order in which the human race has mastered its various
kinds of knowledge, there will arise in every child an aptitude to
acquire these kinds of knowledge in the same order... Education is a
repetition of civilization in little.
— Herbert Spencer
G. Stanley Hall
used Haeckel's theories as the basis for his theories of child
development. His most influential work, "Adolescence: Its Psychology and
Its Relations to Physiology, Anthropology, Sociology, Sex, Crime,
Religion and Education" in 1904
suggested that each individual's life course recapitulated humanity's
evolution from "savagery" to "civilization". Though he has influenced
later childhood development theories, Hall's conception is now generally
considered racist.
Developmental psychologist Jean Piaget favored a weaker version of the formula, according to which ontogeny parallels phylogeny because the two are subject to similar external constraints.
The Austrian pioneer of psychoanalysis, Sigmund Freud,
also favored Haeckel's doctrine. He was trained as a biologist under
the influence of recapitulation theory during its heyday, and retained a
Lamarckian outlook with justification from the recapitulation theory.
Freud also distinguished between physical and mental recapitulation,
in which the differences would become an essential argument for his theory of neuroses.
In the late 20th century, studies of symbolism and learning in
the field of cultural anthropology suggested that "both biological
evolution and the stages in the child's cognitive development follow
much the same progression of evolutionary stages as that suggested in
the archaeological record".
Music criticism
The musicologist Richard Taruskin
in 2005 applied the phrase "ontogeny becomes phylogeny" to the process
of creating and recasting music history, often to assert a perspective
or argument. For example, the peculiar development of the works by
modernist composer Arnold Schoenberg
(here an "ontogeny") is generalized in many histories into a
"phylogeny" – a historical development ("evolution") of Western music
toward atonal styles of which Schoenberg is a representative. Such historiographies
of the "collapse of traditional tonality" are faulted by music
historians as asserting a rhetorical rather than historical point about
tonality's "collapse".
Taruskin also developed a variation of the motto into the pun "ontogeny recapitulates ontology" to refute the concept of "absolute music" advancing the socio-artistic theories of the musicologist Carl Dahlhaus. Ontology
is the investigation of what exactly something is, and Taruskin asserts
that an art object becomes that which society and succeeding
generations made of it. For example, Johann Sebastian Bach's St. John Passion, composed in the 1720s, was appropriated by the Nazi regime in the 1930s for propaganda. Taruskin claims the historical development of the St John Passion (its ontogeny) as a work with an anti-Semitic
message does, in fact, inform the work's identity (its ontology), even
though that was an unlikely concern of the composer. Music or even an
abstract visual artwork can not be truly autonomous ("absolute") because
it is defined by its historical and social reception.
The mediaeval great chain of being as a staircase, implying the possibility of progress: Ramon Lull's Ladder of Ascent and Descent of the Mind, 1305
Alternatives to Darwinian evolution have been proposed by scholars investigating biology to explain signs of evolution and the relatedness
of different groups of living things. The alternatives in question do
not deny that evolutionary changes over time are the origin of the
diversity of life, nor that the organisms alive today share a common
ancestor from the distant past (or ancestors, in some proposals);
rather, they propose alternative mechanisms of evolutionary change over
time, arguing against mutations acted on by natural selection as the
most important driver of evolutionary change.
This distinguishes them from certain other kinds of arguments
that deny that large-scale evolution of any sort has taken place, as in
some forms of creationism,
which do not propose alternative mechanisms of evolutionary change but
instead deny that evolutionary change has taken place at all. Not all
forms of creationism deny that evolutionary change takes place; notably,
proponents of theistic evolution, such as the biologist Asa Gray,
assert that evolutionary change does occur and is responsible for the
history of life on Earth, with the proviso that this process has been
influenced by a god or gods in some meaningful sense.
Where the fact of evolutionary change was accepted but the mechanism proposed by Charles Darwin, natural selection, was denied, explanations of evolution such as Lamarckism, catastrophism, orthogenesis, vitalism, structuralism and mutationism (called saltationism before 1900) were entertained. Different factors motivated people to propose non-Darwinian
mechanisms of evolution. Natural selection, with its emphasis on death
and competition, did not appeal to some naturalists because they felt it
immoral, leaving little room for teleology
or the concept of progress (orthogenesis) in the development of life.
Some who came to accept evolution, but disliked natural selection,
raised religious objections. Others felt that evolution was an
inherently progressive process that natural selection alone was
insufficient to explain. Still others felt that nature, including the
development of life, followed orderly patterns that natural selection
could not explain.
By the start of the 20th century, evolution was generally accepted by biologists but natural selection was in eclipse.
Many alternative theories were proposed, but biologists were quick to
discount theories such as orthogenesis, vitalism and Lamarckism which
offered no mechanism for evolution. Mutationism did propose a mechanism,
but it was not generally accepted. The modern synthesis
a generation later claimed to sweep away all the alternatives to
Darwinian evolution, though some have been revived as molecular
mechanisms for them have been discovered.
Aristotle did not embrace either divine creation or evolution, instead arguing in his biology that each species (eidos) was immutable, breeding true to its ideal eternal form (not the same as Plato's theory of forms). Aristotle's suggestion in De Generatione Animalium of a fixed hierarchy in nature - a scala naturae ("ladder of nature") provided an early explanation of the continuity of living things.Aristotle saw that animals were teleological (functionally end-directed), and had parts that were homologous with those of other animals, but he did not connect these ideas into a concept of evolutionary progress.
In the Middle Ages, Scholasticism developed Aristotle's view into the idea of a great chain of being.
The image of a ladder inherently suggests the possibility of climbing,
but both the ancient Greeks and mediaeval scholastics such as Ramon Lull maintained that each species remained fixed from the moment of its creation.
By 1818, however, Étienne Geoffroy Saint-Hilaire argued in his Philosophie anatomique
that the chain was "a progressive series", where animals like molluscs
low on the chain could "rise, by addition of parts, from the simplicity
of the first formations to the complication of the creatures at the head
of the scale", given sufficient time. Accordingly, Geoffroy and later
biologists looked for explanations of such evolutionary change.
Georges Cuvier's 1812 Recherches sur les Ossements Fossiles
set out his doctrine of the correlation of parts, namely that since an
organism was a whole system, all its parts mutually corresponded,
contributing to the function of the whole. So, from a single bone the
zoologist could often tell what class or even genus the animal belonged
to. And if an animal had teeth adapted for cutting meat, the zoologist
could be sure without even looking that its sense organs would be those
of a predator and its intestines those of a carnivore. A species had an
irreducible functional complexity, and "none of its parts can change
without the others changing too".
Evolutionists expected one part to change at a time, one change to
follow another. In Cuvier's view, evolution was impossible, as any one
change would unbalance the whole delicate system.
Louis Agassiz's
1856 "Essay on Classification" exemplified German philosophical
idealism. This held that each species was complex within itself, had
complex relationships to other organisms, and fitted precisely into its
environment, as a pine tree in a forest, and could not survive outside
those circles. The argument from such ideal forms opposed evolution
without offering an actual alternative mechanism. Richard Owen held a similar view in Britain.
The Lamarckian social philosopher and evolutionist Herbert Spencer, ironically the author of the phrase "survival of the fittest" adopted by Darwin,
used an argument like Cuvier's to oppose natural selection. In 1893, he
stated that a change in any one structure of the body would require all
the other parts to adapt to fit in with the new arrangement. From this,
he argued that it was unlikely that all the changes could appear at the
right moment if each one depended on random variation; whereas in a
Lamarckian world, all the parts would naturally adapt at once, through a
changed pattern of use and disuse.
Where the fact of evolutionary change was accepted by biologists but natural selection was denied, including but not limited to the late 19th century eclipse of Darwinism, alternative scientific explanations such as Lamarckism, orthogenesis, structuralism, catastrophism, vitalism and theistic evolution were entertained, not necessarily separately. (Purely religious points of view such as young or old earth creationism or intelligent design
are not considered here.) Different factors motivated people to propose
non-Darwinian evolutionary mechanisms. Natural selection, with its
emphasis on death and competition, did not appeal to some naturalists
because they felt it immoral, leaving little room for teleology or the concept of progress in the development of life. Some of these scientists and philosophers, like St. George Jackson Mivart and Charles Lyell, who came to accept evolution but disliked natural selection, raised religious objections. Others, such as the biologist and philosopher Herbert Spencer, the botanist George Henslow (son of Darwin's mentor John Stevens Henslow, also a botanist), and the author Samuel Butler,
felt that evolution was an inherently progressive process that natural
selection alone was insufficient to explain. Still others, including the
American paleontologists Edward Drinker Cope and Alpheus Hyatt,
had an idealist perspective and felt that nature, including the
development of life, followed orderly patterns that natural selection
could not explain.
Some felt that natural selection would be too slow, given the estimates of the age of the earth and sun (10–100 million years) being made at the time by physicists such as Lord Kelvin,
and some felt that natural selection could not work because at the time
the models for inheritance involved blending of inherited
characteristics, an objection raised by the engineer Fleeming Jenkin in a review of Origin written shortly after its publication. Another factor at the end of the 19th century was the rise of a new faction of biologists, typified by geneticists like Hugo de Vries and Thomas Hunt Morgan, who wanted to recast biology as an experimental laboratory science. They distrusted the work of naturalists like Darwin and Alfred Russel Wallace, dependent on field observations of variation, adaptation, and biogeography, as being overly anecdotal. Instead they focused on topics like physiology and genetics
that could be investigated with controlled experiments in the
laboratory, and discounted less accessible phenomena like natural
selection and adaptation to the environment.
Vitalism holds that living organisms differ from other things in
containing something non-physical, such as a fluid or vital spirit, that
makes them live. The theory dates to ancient Egypt.
Since Early Modern times, vitalism stood in contrast to the mechanistic explanation of biological systems started by Descartes. Nineteenth century chemists set out to disprove the claim that forming organic compounds required vitalist influence. In 1828, Friedrich Wöhler showed that urea could be made entirely from inorganic chemicals. Louis Pasteur believed that fermentation required whole organisms, which he supposed carried out chemical reactions found only in living things. The embryologist Hans Driesch, experimenting on sea urchin eggs, showed that separating the first two cells led to two complete but small blastulas,
seemingly showing that cell division did not divide the egg into
sub-mechanisms, but created more cells each with the vital capability to
form a new organism. Vitalism faded out with the demonstration of more
satisfactory mechanistic explanations of each of the functions of a
living cell or organism. By 1931, biologists had "almost unanimously abandoned vitalism as an acknowledged belief."
The American botanist Asa Gray used the name "theistic evolution" for his point of view, presented in his 1876 book Essays and Reviews Pertaining to Darwinism. He argued that the deity supplies beneficial mutations to guide evolution. St George Jackson Mivart argued instead in his 1871 On the Genesis of Species
that the deity, equipped with foreknowledge, sets the direction of
evolution by specifying the (orthogenetic) laws that govern it, and
leaves species to evolve according to the conditions they experience as
time goes by. The Duke of Argyll set out similar views in his 1867 book The Reign of Law.
According to the historian Edward Larson, the theory failed as an
explanation in the minds of late 19th century biologists as it broke the
rules of methodological naturalism which they had grown to expect.
Accordingly, by around 1900, biologists no longer saw theistic
evolution as a valid theory. In Larson's view, by then it "did not even
merit a nod among scientists." In the 20th century, theistic evolution could take other forms, such as the orthogenesis of Teilhard de Chardin.
Orthogenesis or Progressionism is the hypothesis that life has an
innate tendency to change, developing in a unilinear fashion in a
particular direction, or simply making some kind of definite progress.
Many different versions have been proposed, some such as that of Teilhard de Chardin openly spiritual, others such as Theodor Eimer's
apparently simply biological. These theories often combined
orthogenesis with other supposed mechanisms. For example, Eimer believed
in Lamarckian evolution, but felt that internal laws of growth
determined which characteristics would be acquired and would guide the
long-term direction of evolution.
Orthogenesis was popular among paleontologists such as Henry Fairfield Osborn.
They believed that the fossil record showed unidirectional change, but
did not necessarily accept that the mechanism driving orthogenesis was teleological (goal-directed). Osborn argued in his 1918 book Origin and Evolution of Life that trends in Titanothere horns were both orthogenetic and non-adaptive, and could be detrimental to the organism. For instance, they supposed that the large antlers of the Irish elk had caused its extinction.
Support for orthogenesis fell during the modern synthesis
in the 1940s when it became apparent that it could not explain the
complex branching patterns of evolution revealed by statistical analysis
of the fossil record.
Work in the 21st century has supported the mechanism and existence of
mutation-biased adaptation (a form of mutationism), meaning that
constrained orthogenesis is now seen as possible. Moreover, the self-organizing processes involved in certain aspects of embryonic development
often exhibit stereotypical morphological outcomes, suggesting that
evolution will proceed in preferred directions once key molecular
components are in place.
Jean-Baptiste Lamarck's 1809 evolutionary theory, transmutation of species,
was based on a progressive (orthogenetic) drive toward greater
complexity. Lamarck also shared the belief, common at the time, that characteristics acquired during an organism's life could be inherited
by the next generation, producing adaptation to the environment. Such
characteristics were caused by the use or disuse of the affected part of
the body. This minor component of Lamarck's theory became known, much
later, as Lamarckism. Darwin included Effects of the increased Use and Disuse of Parts, as controlled by Natural Selection in On the Origin of Species,
giving examples such as large ground feeding birds getting stronger
legs through exercise, and weaker wings from not flying until, like the ostrich, they could not fly at all. In the late 19th century, neo-Lamarckism was supported by the German biologist Ernst Haeckel, the American paleontologistsEdward Drinker Cope and Alpheus Hyatt, and the American entomologistAlpheus Packard. Butler and Cope believed that this allowed organisms to effectively drive their own evolution.
Packard argued that the loss of vision in the blind cave insects he
studied was best explained through a Lamarckian process of atrophy
through disuse combined with inheritance of acquired characteristics. Meanwhile, the English botanist George Henslow
studied how environmental stress affected the development of plants,
and he wrote that the variations induced by such environmental factors
could largely explain evolution; he did not see the need to demonstrate
that such variations could actually be inherited.
Critics pointed out that there was no solid evidence for the
inheritance of acquired characteristics. Instead, the experimental work
of the German biologist August Weismann
resulted in the germ plasm theory of inheritance, which Weismann said
made the inheritance of acquired characteristics impossible, since the Weismann barrier would prevent any changes that occurred to the body after birth from being inherited by the next generation.
Catastrophism is the hypothesis, argued by the French anatomist and paleontologistGeorges Cuvier in his 1812 Recherches sur les ossements fossiles de quadrupèdes, that the various extinctions and the patterns of faunal succession seen in the fossil
record were caused by large-scale natural catastrophes such as volcanic
eruptions and, for the most recent extinctions in Eurasia, the
inundation of low-lying areas by the sea. This was explained purely by natural events: he did not mention Noah's flood,
nor did he ever refer to divine creation as the mechanism for
repopulation after an extinction event, though he did not support
evolutionary theories such as those of his contemporaries Lamarck and
Geoffroy Saint-Hilaire either. Cuvier believed that the stratigraphic
record indicated that there had been several such catastrophes,
recurring natural events, separated by long periods of stability during
the history of life on earth. This led him to believe the Earth was
several million years old.
Biological structuralism objects to an exclusively Darwinian
explanation of natural selection, arguing that other mechanisms also
guide evolution, and sometimes implying that these supersede selection
altogether. Structuralists have proposed different mechanisms that might have guided the formation of body plans. Before Darwin, Étienne Geoffroy Saint-Hilaire argued that animals shared homologous parts, and that if one was enlarged, the others would be reduced in compensation. After Darwin, D'Arcy Thompson hinted at vitalism and offered geometric explanations in his classic 1917 book On Growth and Form. Adolf Seilacher suggested mechanical inflation for "pneu" structures in Ediacaran biota fossils such as Dickinsonia. Günter P. Wagner argued for developmental bias, structural constraints on embryonic development. Stuart Kauffman favoured self-organisation, the idea that complex structure emerges holistically and spontaneously from the dynamic interaction of all parts of an organism. Michael Denton argued for laws of form by which Platonic universals or "Types" are self-organised. In 1979 Stephen J. Gould and Richard Lewontin proposed biological "spandrels", features created as a byproduct of the adaptation of nearby structures. Gerd Müller and Stuart Newman argued that the appearance in the fossil record of most of the current phyla in the Cambrian explosion was "pre-Mendelian" evolution caused by plastic responses of morphogenetic systems that were partly organized by physical mechanisms. Brian Goodwin, described by Wagner as part of "a fringe movement in evolutionary biology", denied that biological complexity can be reduced to natural selection, and argued that pattern formation is driven by morphogenetic fields. Darwinian biologists have criticised structuralism, emphasising that there is plentiful evidence from deep homology that genes have been involved in shaping organisms throughout evolutionary history. They accept that some structures such as the cell membrane self-assemble, but question the ability of self-organisation to drive large-scale evolution.
Saltationism held that new species arise as a result of large mutations.
It was seen as a much faster alternative to the Darwinian concept of a
gradual process of small random variations being acted on by natural
selection. It was popular with early geneticists such as Hugo de Vries, who along with Carl Correns helped rediscover Gregor Mendel's laws of inheritance in 1900, William Bateson, a British zoologist who switched to genetics, and early in his career, Thomas Hunt Morgan. These ideas developed into mutationism, the mutation theory of evolution.
This held that species went through periods of rapid mutation, possibly
as a result of environmental stress, that could produce multiple
mutations, and in some cases completely new species, in a single
generation, based on de Vries's experiments with the evening primrose, Oenothera,
from 1886. The primroses seemed to be constantly producing new
varieties with striking variations in form and color, some of which
appeared to be new species because plants of the new generation could
only be crossed with one another, not with their parents. However, Hermann Joseph Muller showed in 1918 that the new varieties de Vries had observed were the result of polyploid hybrids rather than rapid genetic mutation.
Initially, de Vries and Morgan believed that mutations were so
large as to create new forms such as subspecies or even species
instantly. Morgan's 1910 fruit fly experiments, in which he isolated
mutations for characteristics such as white eyes, changed his mind. He
saw that mutations represented small Mendelian
characteristics that would only spread through a population when they
were beneficial, helped by natural selection. This represented the germ
of the modern synthesis, and the beginning of the end for mutationism as an evolutionary force.
Contemporary biologists accept that mutation and selection both
play roles in evolution; the mainstream view is that while mutation
supplies material for selection in the form of variation, all non-random
outcomes are caused by natural selection. Masatoshi Nei
argues instead that the production of more efficient genotypes by
mutation is fundamental for evolution, and that evolution is often
mutation-limited. The endosymbiotic theory implies rare but major events of saltational evolution by symbiogenesis. Carl Woese and colleagues suggested that the absence of RNA signature continuum between domains of bacteria, archaea, and eukarya shows that these major lineages materialized via large saltations in cellular organization. Saltation at a variety of scales is agreed to be possible by mechanisms including polyploidy, which certainly can create new species of plant, gene duplication, lateral gene transfer, and transposable elements (jumping genes).
The neutral theory of molecular evolution, proposed by Motoo Kimura in 1968, holds that at the molecular level most evolutionary changes and most of the variation within and between species is not caused by natural selection but by genetic drift of mutantalleles that are neutral. A neutral mutation
is one that does not affect an organism's ability to survive and
reproduce. The neutral theory allows for the possibility that most
mutations are deleterious, but holds that because these are rapidly
purged by natural selection, they do not make significant contributions
to variation within and between species at the molecular level.
Mutations that are not deleterious are assumed to be mostly neutral
rather than beneficial.
The theory was controversial as it sounded like a challenge to
Darwinian evolution; controversy was intensified by a 1969 paper by Jack Lester King and Thomas H. Jukes, provocatively but misleadingly titled "Non-Darwinian Evolution". It provided a wide variety of evidence including protein sequence comparisons, studies of the Treffers mutator gene in E. coli, analysis of the genetic code, and comparative immunology, to argue that most protein evolution is due to neutral mutations and genetic drift.
According to Kimura, the theory applies only for evolution at the molecular level, while phenotypic evolution is controlled by natural selection, so the neutral theory does not constitute a true alternative.
Combined theories
Multiple
explanations have been offered since the 19th century for how evolution
took place, given that many scientists initially had objections to
natural selection (dashed orange arrows). Many of these theories led
(blue arrows) to some form of directed evolution (orthogenesis), with or without invoking divine control (dotted blue arrows) directly or indirectly. For example, evolutionists like Edward Drinker Cope believed in a combination of theistic evolution, Lamarckism, vitalism, and orthogenesis, represented by the sequence of arrows on the extreme left of the diagram.
The various alternatives to Darwinian evolution by natural selection
were not necessarily mutually exclusive. The evolutionary philosophy of
the American palaeontologist Edward Drinker Cope
is a case in point. Cope, a religious man, began his career denying the
possibility of evolution. In the 1860s, he accepted that evolution
could occur, but, influenced by Agassiz, rejected natural selection.
Cope accepted instead the theory of recapitulation of evolutionary
history during the growth of the embryo - that ontogeny recapitulates phylogeny, which Agassiz believed showed a divine plan leading straight up to man, in a pattern revealed both in embryology and palaeontology.
Cope did not go so far, seeing that evolution created a branching tree
of forms, as Darwin had suggested. Each evolutionary step was however
non-random: the direction was determined in advance and had a regular
pattern (orthogenesis), and steps were not adaptive but part of a divine
plan (theistic evolution). This left unanswered the question of why
each step should occur, and Cope switched his theory to accommodate
functional adaptation for each change. Still rejecting natural selection
as the cause of adaptation, Cope turned to Lamarckism to provide the
force guiding evolution. Finally, Cope supposed that Lamarckian use and
disuse operated by causing a vitalist growth-force substance,
"bathmism", to be concentrated in the areas of the body being most
intensively used; in turn, it made these areas develop at the expense of
the rest. Cope's complex set of beliefs thus assembled five
evolutionary philosophies: recapitulationism, orthogenesis, theistic
evolution, Lamarckism, and vitalism.
Other palaeontologists and field naturalists continued to hold beliefs
combining orthogenesis and Lamarckism until the modern synthesis in the
1930s.
Rebirth of natural selection, with continuing alternatives
By the start of the 20th century, during the eclipse of Darwinism,
biologists were doubtful of natural selection, but equally were quick
to discount theories such as orthogenesis, vitalism and Lamarckism which
offered no mechanism for evolution. Mutationism did propose a
mechanism, but it was not generally accepted. The modern synthesis
a generation later, roughly between 1918 and 1932, broadly swept away
all the alternatives to Darwinism, though some including forms of
orthogenesis, epigenetic mechanisms that resemble Lamarckian inheritance of acquired characteristics, catastrophism, structuralism, and mutationism have been revived, such as through the discovery of molecular mechanisms.
Biology has become Darwinian, but belief in some form of progress
(orthogenesis) remains both in the public mind and among biologists.
Ruse argues that evolutionary biologists will probably continue to
believe in progress for three reasons. Firstly, the anthropic principle
demands people able to ask about the process that led to their own
existence, as if they were the pinnacle of such progress. Secondly,
scientists in general and evolutionists in particular believe that their
work is leading them progressively closer to a true grasp of reality, as knowledge increases, and hence (runs the argument) there is progress in nature also. Ruse notes in this regard that Richard Dawkins explicitly compares cultural progress with memes
to biological progress with genes. Thirdly, evolutionists are
self-selected; they are people, such as the entomologist and
sociobiologist E. O. Wilson, who are interested in progress to supply a meaning for life.
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