Steady state (biochemistry)

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https://en.wikipedia.org/wiki/Steady_state_(biochemistry)

In biochemistry, steady state refers to the maintenance of constant internal concentrations of molecules and ions in the cells and organs of living systems. Living organisms remain at a dynamic steady state where their internal composition at both cellular and gross levels are relatively constant, but different from equilibrium concentrations. A continuous flux of mass and energy results in the constant synthesis and breakdown of molecules via chemical reactions of biochemical pathways. Essentially, steady state can be thought of as homeostasis at a cellular level.

Maintenance of steady state

Figure 1: For a metabolic pathway at steady state, the rate of input into the pathway, the rate of conversion of A to B and the rate of output are all equal, allowing intermediates A and B to maintain constant steady state concentrations.

Metabolic regulation achieves a balance between the rate of input of a substrate and the rate that it is degraded or converted, and thus maintains steady state. The rate of metabolic flow, or flux, is variable and subject to metabolic demands. However, in a metabolic pathway, steady state is maintained by balancing the rate of substrate provided by a previous step and the rate that the substrate is converted into product, keeping substrate concentration relatively constant.

Thermodynamically speaking, living organisms are open systems, meaning that they constantly exchange matter and energy with their surroundings. A constant supply of energy is required for maintaining steady state, as maintaining a constant concentration of a molecule preserves internal order and thus is entropically unfavorable. When a cell dies and no longer utilizes energy, its internal composition will proceed toward equilibrium with its surroundings.

In some occurrences, it is necessary for cells to adjust their internal composition in order to reach a new steady state. Cell differentiation, for example, requires specific protein regulation that allows the differentiating cell to meet new metabolic requirements.

ATP

The concentration of ATP must be kept above equilibrium level so that the rates of ATP-dependent biochemical reactions meet metabolic demands. A decrease in ATP will result in a decreased saturation of enzymes that use ATP as substrate, and thus a decreased reaction rate. The concentration of ATP is also kept higher than that of AMP, and a decrease in the ATP/AMP ratio triggers AMPK to activate cellular processes that will return ATP and AMP concentrations to steady state.

In one step of the glycolysis pathway catalyzed by PFK-1, the equilibrium constant of reaction is approximately 1000, but the steady state concentration of products (fructose-1,6-bisphosphate and ADP) over reactants (fructose-6-phosphate and ATP) is only 0.1, indicating that the ratio of ATP to AMP remains in a steady state significantly above equilibrium concentration. Regulation of PFK-1 maintains ATP levels above equilibrium.

In the cytoplasm of hepatocytes, the steady state ratio of NADP⁺ to NADPH is approximately 0.1 while that of NAD⁺ to NADH is approximately 1000, favoring NADPH as the main reducing agent and NAD⁺ as the main oxidizing agent in chemical reactions.

Blood glucose

Figure 2: Regulation of metabolic pathways maintains blood glucose concentration at approximately 5 mM in humans.

Blood glucose levels are maintained at a steady state concentration by balancing the rate of entry of glucose into the blood stream (i.e. by ingestion or released from cells) and the rate of glucose uptake by body tissues. Changes in the rate of input will be met with a change in consumption, and vice versa, so that blood glucose concentration is held at about 5 mM in humans. A change in blood glucose levels triggers the release of insulin or glucagon, which stimulates the liver to release glucose into the bloodstream or take up glucose from the bloodstream in order to return glucose levels to steady state. Pancreatic beta cells, for example, increase oxidative metabolism as a result of a rise in blood glucose concentration, triggering secretion of insulin. Glucose levels in the brain are also maintained at steady state, and glucose delivery to the brain relies on the balance between the flux of the blood brain barrier and uptake by brain cells. In teleosts, a drop of blood glucose levels below that of steady state decreases the intracellular-extracellular gradient in the bloodstream, limiting glucose metabolism in red blood cells.

Blood lactate

Blood lactate levels are also maintained at steady state. At rest or low levels of exercise, the rate of lactate production in muscle cells and consumption in muscle or blood cells allows lactate to remain in the body at a certain steady state concentration. If a higher level of exercise is sustained, however, blood lactose levels will increase before becoming constant, indicating that a new steady state of elevated concentration has been reached. Maximal lactate steady state (MLSS) refers to the maximum constant concentration of lactase reached during sustained high-activity.

Nitrogen-containing molecules

Metabolic regulation of nitrogen-containing molecules, such as amino acids, is also kept at steady state. The amino acid pool, which describes the level of amino acids in the body, is maintained at a relatively constant concentration by balancing the rate of input (i.e. from dietary protein ingestion, production of metabolic intermediates) and rate of depletion (i.e. from formation of body proteins, conversion to energy-storage molecules). Amino acid concentration in lymph node cells, for example, is kept at steady state with active transport as the primary source of entry, and diffusion as the source of efflux.

Ions

Figure 3: Different steady state concentrations of ions on either side of the cell membrane maintain a resting membrane potential.

One main function of plasma and cell membranes is to maintain asymmetric concentrations of inorganic ions in order to maintain an ionic steady state different from electrochemical equilibrium. In other words, there is a differential distribution of ions on either side of the cell membrane - that is, the amount of ions on either side is not equal and therefore a charge separation exists. However, ions move across the cell membrane such that a constant resting membrane potential is achieved; this is ionic steady state. In the pump-leak model of cellular ion homeostasis, energy is utilized to actively transport ions against their electrochemical gradient. The maintenance of this steady state gradient, in turn, is used to do electrical and chemical work, when it is dissipated though the passive movement of ions across the membrane.

In cardiac muscle, ATP is used to actively transport sodium ions out of the cell through a membrane ATPase. Electrical excitation of the cell results in an influx of sodium ions into the cell, temporarily depolarizing the cell. To restore the steady state electrochemical gradient, ATPase removes sodium ions and restores potassium ions in the cell. When an elevated heart rate is sustained, causing more depolarizations, sodium levels in the cell increase until becoming constant, indicating that a new steady state has been reached.

Stability of the steady-state

Steady-states can be stable or unstable. A steady-state is unstable if a small perturbation in one or more of the concentrations results in the system diverging from its state. In contrast, if a steady-state is stable, any perturbation will relax back to the original steady state. Further details can be found on the page Stability theory.

Simple Example

The following provides a simple example for computing the steady-state give a simple mathematical model.

Consider the open chemical system composed of two reactions with rates ${\displaystyle v_1}$and ${\displaystyle v_2:}$:

${\displaystyle X_o\stackrel{v_1}{⟶}{S}_1\stackrel{v_2}{⟶}{X}_1}$

We will assume that the chemical species ${\displaystyle X_o}$ and ${\displaystyle X_1}$ are fixed external species and ${\displaystyle S_1}$ is an internal chemical species that is allowed to change. The fixed boundaries is to ensure the system can reach a steady-state. If we assume simple irreversible mass-action kinetics, the differential equation describing the concentration of ${\displaystyle S_1}$ is given by:

${\displaystyle \frac{d{S}_1}{dt}=k_1{X}_o-k_2{S}_1}$

To find the steady-state the differential equation is set to zero and the equation rearranged to solve for ${\displaystyle S_1}$

${\displaystyle S_1=\frac{k_1{X}_o}{k_2}}$

This is the steady-state concentration of ${\displaystyle S_1}$.

The stability of this system can be determined by making a perturbation ${\displaystyle \delta S_1}$ in ${\displaystyle S_1}$ This can be expressed as:

${\displaystyle \frac{d(S_1+\delta S_1)}{dt}=k_1{X}_o-k_2(S_1+\delta S_1)}$

Note that the ${\displaystyle \delta S_1}$ will elicit a change in the rate of change. At steady-state ${\displaystyle k_1{X}_o-k_2{S}_1=0}$, therefore the rate of change of ${\displaystyle S_1}$ as a result of this perturbation is:

${\displaystyle \frac{d\delta S_1}{dt}=-k_2\delta S_1}$

This shows that the perturbation, ${\displaystyle \delta S_1}$ decays exponetially, hence the system is stable.

Great Red Spot

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Great_Red_Spot

Close up view of the Great Red Spot by Juno

The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter, producing an anticyclonic storm that is the largest in the Solar System. It is the most recognizable feature on Jupiter, owing to its red-orange color whose origin is still unknown. Located 22 degrees south of Jupiter's equator, it produces wind-speeds up to 432 km/h (268 mph). It was first observed in September 1831, with 60 recorded observations between then and 1878, when continuous observations began. A similar spot was observed from 1665 to 1713; if this is the same storm, it has existed for at least 360 years, but a study from 2024 suggests this is not the case.

Observation history

First observations

Donato Creti's 1711 painting "Jupiter", the first depiction of the Great Red Spot as red

A sketch of Jupiter made by Thomas Gwyn Elger in November 1881, showing the Great Red Spot

The Great Red Spot may have existed before 1665, but it could be that the present spot was first seen only in 1830, and was well studied only after a prominent appearance in 1879. The storm that was seen in the 17th century may have been different from the storm that exists today. A long gap separates its period of current study after 1830 from its 17th century discovery. Whether the original spot dissipated and reformed, whether it faded, or if the observational record was simply poor is unknown.

The first sighting of the Great Red Spot is often credited to Robert Hooke, who described a spot on the planet in May 1664. However, it is likely that Hooke's spot was not only in another belt altogether (the North Equatorial Belt, as opposed to the current Great Red Spot's location in the South Equatorial Belt), but also that it was in the shadow of a transiting moon, most likely Callisto. Far more convincing is Giovanni Cassini's description of a "permanent spot" the following year. With fluctuations in visibility, Cassini's spot was observed from 1665 to 1713, but the 48-year observational gap makes the identity of the two spots inconclusive. The older spot's shorter observational history and slower motion than the modern spot makes it difficult to conclude that they are the same.

A minor mystery concerns a Jovian spot depicted in a 1711 canvas by Donato Creti, which is exhibited in the Vatican. Part of a series of panels in which different (magnified) heavenly bodies serve as backdrops for various Italian scenes, and all overseen by the astronomer Eustachio Manfredi for accuracy, Creti's painting is the first known depiction of the Great Red Spot as red (albeit raised to the Jovian northern hemisphere due to an optical inversion inherent to the era's telescopes). No Jovian feature was explicitly described in writing as red before the late 19th century.

The Great Red Spot has been observed since 5 September 1831. By 1879, over 60 observations had been recorded. Since it came into prominence in 1879, it has been under continuous observation.

A 2024 study of historical observations suggests that the "permanent spot" observed from 1665 to 1713 may not be the same as the modern Great Red Spot observed since 1831. It is suggested that the original spot disappeared, and later another spot formed, which is the one seen today.

Late 20th and 21st centuries

A wide view of Jupiter and the Great Red Spot as seen from Voyager 1 in 1979.

On 25 February 1979, when the Voyager 1 spacecraft was 9,200,000 km (5,700,000 mi) from Jupiter, it transmitted the first detailed image of the Great Red Spot. Cloud details as small as 160 km (100 mi) across were visible. The colorful, wavy cloud pattern seen to the left (west) of the Red Spot is a region of extraordinarily complex and variable wave motion.

In the 21st century, the major diameter of the Great Red Spot has been observed to be shrinking in size. At the start of 2004, its length was about half that of a century earlier, when it reached a size of 40,000 km (25,000 mi), about three times the diameter of Earth. At the present rate of reduction, it will become circular by 2040. It is not known how long the spot will last, or whether the change is a result of normal fluctuations. In 2019, the Great Red Spot began "flaking" at its edge, with fragments of the storm breaking off and dissipating. The shrinking and "flaking" fueled speculation from some astronomers that the Great Red Spot could dissipate within 20 years. However, other astronomers believe that the apparent size of the Great Red Spot reflects its cloud coverage and not the size of the actual, underlying vortex, and they also believe that the flaking events can be explained by interactions with other cyclones or anticyclones, including incomplete absorptions of smaller systems; if this is the case, this would mean that the Great Red Spot is not in danger of dissipating.

A smaller spot, designated Oval BA, which formed in March 2000 from the merging of three white ovals, has turned reddish in color. Astronomers have named it the Little Red Spot or Red Jr. As of 5 June 2006, the Great Red Spot and Oval BA appeared to be approaching convergence. The storms pass each other about every two years, but the passings of 2002 and 2004 were of little significance. Amy Simon-Miller, of the Goddard Space Flight Center, predicted the storms would have their closest passing on 4 July 2006. She worked with Imke de Pater and Phil Marcus of UC Berkeley as well as a team of professional astronomers beginning in April 2006 to study the storms using the Hubble Space Telescope; on 20 July 2006, the two storms were photographed passing each other by the Gemini Observatory without converging. In May 2008, a third storm turned red.

The Juno spacecraft, which entered into a polar orbit around Jupiter in 2016, flew over the Great Red Spot upon its close approach to Jupiter on 11 July 2017, taking several images of the storm from a distance of about 8,000 km (5,000 mi) above the surface. Over the duration of the Juno mission, the spacecraft continued to study the composition and evolution of Jupiter's atmosphere, especially its Great Red Spot.

The Great Red Spot should not be confused with the Great Dark Spot, a feature observed near the northern pole of Jupiter in 2000 with the Cassini–Huygens spacecraft. There is also a feature in the atmosphere of Neptune called the Great Dark Spot. The latter feature was imaged by Voyager 2 in 1989 and may have been an atmospheric hole rather than a storm. It was no longer present as of 1994, although a similar spot had appeared farther to the north.

Mechanical dynamics

Time-lapse sequence from the approach of Voyager 1 to Jupiter in 1979, showing the motion of atmospheric bands, and the circulation of the Great Red Spot. The black spots that appear are shadows cast by Jupiter's moons.

Jupiter's Great Red Spot rotates counterclockwise, with a period of about 4.5 Earth days, or 11 Jovian days, as of 2008. Measuring 16,350 km (10,160 mi) in width as of 3 April 2017, the Great Red Spot is 1.3 times the diameter of Earth. The cloud-tops of this storm are about 8 km (5 mi) above the surrounding cloud-tops. The storm has continued to exist for centuries because there is no planetary surface (only a mantle of hydrogen) to provide friction; circulating gas eddies persist for a very long time in the atmosphere because there is nothing to oppose their angular momentum.

Infrared data has long indicated that the Great Red Spot is colder (and thus higher in altitude) than most of the other clouds on the planet. The upper atmosphere above the storm, however, has substantially higher temperatures than the rest of the planet. Acoustic (sound) waves rising from the turbulence of the storm below have been proposed as an explanation for the heating of this region. The acoustic waves travel vertically up to a height of 800 km (500 mi) above the storm where they break in the upper atmosphere, converting wave energy into heat. This creates a region of upper atmosphere that is 1,600 K (1,330 °C; 2,420 °F)—several hundred kelvins warmer than the rest of the planet at this altitude. The effect is described as like "crashing [...] ocean waves on a beach".

Size of the Earth compared to the Great Red Spot

Careful tracking of atmospheric features revealed the Great Red Spot's counterclockwise circulation as far back as 1966, observations dramatically confirmed by the first time-lapse movies from the Voyager fly-bys. The spot is confined by a modest eastward jet stream to its south and a very strong westward one to its north. Though winds around the edge of the spot peak at about 432 km/h (268 mph), currents inside it seem stagnant, with little inflow or outflow. The rotation period of the spot has decreased with time, perhaps as a direct result of its steady reduction in size.

The Great Red Spot's latitude has been stable for the duration of good observational records, typically varying by about a degree. Its longitude, however, is subject to constant variation, including a 90-day longitudinal oscillation with an amplitude of ~1°. Because Jupiter does not rotate uniformly at all latitudes, astronomers have defined three different systems for defining longitude. System II is used for latitudes of more than 10 degrees and was originally based on the average rotational period of the Great Red Spot of 9h 55m 42s. Despite this, however, the spot has "lapped" the planet in System II at least 10 times since the early 19th century. Its drift rate has changed dramatically over the years and has been linked to the brightness of the South Equatorial Belt and the presence or absence of a South Tropical Disturbance.

Internal depth and structure

Clockwise from top left: Hubble image of visible spectrum; infrared from the Gemini Observatory; multiwavelength composite of Hubble and Gemini data showing visible light in blue and thermal infrared in red; ultraviolet image from Hubble; visible light detail 

Jupiter's Great Red Spot (GRS) is an elliptical shaped anticyclone, occurring at 22 degrees below the equator, in Jupiter's southern hemisphere. The largest anticyclonic storm (~16,000 km) in our solar system, little is known about its internal depth and structure. Visible imaging and cloud-tracking from in-situ observation determined the velocity and vorticity of the GRS, which is located in a thin anticyclonic ring at 70–85% of the radius and is located along Jupiter's fastest westward moving jet stream. During NASA's 2016 Juno mission, gravity signature and thermal infrared data were obtained that offered insight into the structural dynamics and depth of the GRS. During July 2017, the Juno spacecraft conducted a second pass of the GRS to collect Microwave Radiometer (MWR) scans of the GRS to determine how far the GRS extended toward the surface of the condensed H₂O layer. These MWR scans suggested that the GRS vertical depth extended to about 240 km below cloud level, with an estimated drop in atmospheric pressure to 100 bar. Two methods of analysis that constrain the data collected were the mascon approach, which found a depth of ~290 km, and the Slepian approach showing wind extending to ~310 km. These methods, along with gravity signature MWR data, suggest that the GRS zonal winds still increase at a rate of 50% of the velocity of the viable cloud level, before the wind decay starts at lower levels. This rate of wind decay and gravity data suggest the depth of the GRS is between 200 and 500 km.

Galileo and Cassini's thermal infrared imaging and spectroscopy of the GRS were conducted during 1995–2008, in order to find evidence of thermal inhomogeneities within the internal structure vortex of the GRS. Previous thermal infrared temperature maps from the Voyager, Galileo, and Cassini missions suggested the GRS is a structure of an anticyclonic vortex with a cold core within a upwelling warmer annulus; this data shows a gradient in the temperature of the GRS. Better understanding of Jupiter's atmospheric temperature, aerosol particle opacity, and ammonia gas composition was provided by thermal-IR imaging: a direct correlation of the visible cloud layers reactions, thermal gradient and compositional mapping to observational data were collected over decades. During December 2000, high spatial resolution images from Galileo, of an atmospheric turbulent area to the northwest of the GRS, showed a thermal contrast between the warmest region of the anticyclone and regions to the east and west of the GRS.

Winds in the Great Red Spot as analyzed from Hubble's data. Red means faster wind, blue means slower wind. 

The vertical temperature of the structure of the GRS is constrained between the 100–600 mbar range, with the vertical temperature of the GRS core at approximately 400 mbar of pressure^{[clarification needed]} being 1.0–1.5°K, much warmer than regions of the GRS to the east–west, and 3.0–3.5°K warmer than regions to the north–south of the structure's edge. This structure is consistent with the data collected by the VISIR (VLT Mid-Infrared Imager Spectrometer on the ESO Very Large Telescope) imaging obtained in 2006; this revealed that the GRS was physically present at a wide range of altitudes that occur within the atmospheric pressure range of 80–600 mbar, and confirms the thermal infrared mapping result. To develop a model of the internal structure of the GRS, the Cassini instrument Composite Infrared Spectrometer (CIRS) and ground based spatial imaging mapped the composition of the phosphine and ammonia aerosols (PH₃, NH₃) and para-hydroxybenzoic acid within the anticyclonic circulation of the GRS. The images that were collected from the CIRS and ground-based imaging trace the vertical motion in the Jovian atmosphere by PH₃ and NH₃ spectra.

The highest concentrations of PH₃ and NH₃ were found to the north of the GRS peripheral rotation. They aided in determining the southward jet movement and showed evidence of an increase in altitude of the column of aerosols with pressures ranging from 200–500 mbar. However, the NH₃ composition data shows that there is a major depletion of NH₃ below the visible cloud layer at the southern peripheral ring of the GRS; this lower opacity is relative to a narrow band of atmospheric subsidence. The low mid-IR aerosol opacity, along with the temperature gradients, the altitude difference, and the vertical movement of the zonal winds, are involved with the development and sustainability of the vorticity. The stronger atmospheric subsidence and compositional asymmetries of the GRS suggest that the structure exhibits a degree of tilt from the northern edge to the southern edge of the structure. The GRS depth and internal structure has been constantly changing over decades; however there is still no logical reason that it is 200–500 km in depth, but the jet streams that supply the force that powers the GRS vortex are well below the structure base.

Color and composition

It is not known what causes the Great Red Spot's reddish color. Hypotheses supported by laboratory experiments suppose that it may be caused by chemical products created from the solar ultraviolet irradiation of ammonium hydrosulfide and the organic compound acetylene, which produces a reddish material—likely complex organic compounds called tholins. The high altitude of the compounds may also contribute to the coloring.

The Great Red Spot varies greatly in hue, from almost brick-red to pale salmon or even white. The spot occasionally disappears, becoming evident only through the Red Spot Hollow, which is its location in the South Equatorial Belt (SEB). Its visibility is apparently coupled to the SEB; when the belt is bright white, the spot tends to be dark, and when it is dark, the spot is usually light. These periods when the spot is dark or light occur at irregular intervals; from 1947 to 1997, the spot was darkest in the periods 1961–1966, 1968–1975, 1989–1990, and 1992–1993.

Contingency (evolutionary biology)

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Contingency_(evolutionary_biology)

In evolutionary biology, contingency describes how the outcome of evolution may be affected by the history of a particular lineage.

Overview

Evolution is a historical process, and the outcomes of history can be sensitive to the details of the interactions and events that preceded them. Contingency was especially emphasized by Stephen Jay Gould, particularly in his 1989 book Wonderful Life. Gould used the thought experiment of rewinding the "tape of life" to the distant past, and argued that even small changes to history would result in evolutionary outcomes very different from our world. Gould's thought experiment has inspired real experiments in the lab and in the field, as well as study of living and extinct organisms as natural experiments.

These studies have found that repeatability in evolution is common, particularly in cases of similar founding populations, when defining repeatability broadly, and over the timescales observable in experiments. Convergent evolution has also been found to be unexpectedly widespread in nature, though it occurs more often among closely related taxa that share more genes and developmental biases, indicating that contingency and convergence may both play a role. Additionally, a trait may be convergent at a broader level of description while being divergent at a more detailed level, with an example being the differently structured wings of insects, pterosaurs, birds, and bats. Knowing how common convergence is also requires more research into how often a trait failed to evolve under the same selective pressures, as well as into traits that evolved only once among all known organisms.

Some examples of contingency affecting evolutionary outcomes have been identified. In the E. coli long-term evolution experiment, out of the 12 populations, only one evolved the highly beneficial trait of growing on citrate, which further experimental replays using frozen ancestral bacteria showed required particular 'potentiating' mutations to arise first. Woodpeckers and aye-ayes occupy the same ecological niche of locating and extracting beetle larvae from wood, but do so by very different means (beak and elongated finger respectively) due to their respective evolutionary histories, as birds lack fingers and primates lack beaks. The unique flora and fauna of isolated locations on Earth, such as New Zealand, as well as from extinct lineages such as the non-avian dinosaurs during the Mesozoic, are also examples of contingency in evolution resulting in different outcomes.

In Wonderful Life

Replaying the "tape of life"

The central question proposed by Wonderful Life is that if life initially proliferated into a greater variety of phyla than currently exist and were subsequently decimated by the stochastic grim reaper of extinction, what then can be said about the inevitability of human intelligence and superiority? Additionally, Gould asks what role historical contingencies play in the evolution of life on Earth. It is these central ideas which prompt Gould to propose a thought experiment called "replaying the tape of life." Its central essence is this: if we rewind the clock and replay the history of life on Earth numerous times, will we consistently see the same outcome that is the reality we experience today? The outcome of this thought experiment has two possible interpretations, elaborated by Gould,

"Suppose that ten of a hundred designs will survive and diversify. If the ten survivors are predictable by superiority of anatomy (Interpretation 1), then they will win each time – and Burgess eliminations do not challenge our comforting view of life. But if the ten survivors are protégés of Lady Luck or fortunate beneficiaries of odd historical contingencies (Interpretation 2), then each replay of the tape will yield a different set of survivors and a radically different history. And if you recall from high-school algebra how to calculate permutations and combinations, you will realize that the total number of combinations for 10 items from a pool of 100 yields more than 17 trillion potential outcomes." – Stephen Jay Gould

Gould's opinion, and the central argument of Wonderful Life, is that "any replay of the tape of life would lead evolution down a pathway radically different from the road actually taken." Additionally, Gould argues, no outcome can be predicated from the start, but the resulting pattern that emerges after replaying the tape of life would be just as interpretable and logical as our current situation.

Evolutionary iconography

In Wonderful Life, Stephen Jay Gould discusses the iconography of evolution in popular culture and the damaging effects of the march of progress on public understanding of the theory.

The march of progress, Gould argues, has led to the popular interpretation that the evolution of increased mental powers, ultimately culminating in the development of man's complex brain, is the natural outcome of evolution. Thus, the term "Evolution" is often conflated with a linear progression of life towards ever-increasing mental powers and a "comfortable view of human inevitability and superiority." Gould argues that the definition of Evolution to professional biologists is "adaptation to changing environments", not progress, and that the composition of life on the planet is rather a "copiously branching bush, continually pruned by the grim reaper of extinction, not a ladder of predictable progress." He discusses society's obsession with unsuccessful lineages as "textbook cases" of "evolution". To elaborate, we consistently seek out "a single line of advance from the true topology of copious branching. In this misguided effort, we are inevitably drawn to branches so near the brink of total annihilation that they retain only one surviving twig. We then view this twig as the acme of upward achievement, rather than the probable last grasp of a richer ancestry." Gould uses the evolution of the horse to illustrate this point, as the unbroken connection between Hyracotherium (formerly called Eohippus) and Equus provides an apparent linear path from simplicity to complexity. The only reason the evolution of horses has become the canonical representation of progressive evolution is because their bush has been extremely unsuccessful. Instead, Gould argues, we should look to bats, antelopes, and rodents as champions of mammalian evolution as they present us with "thousands of twigs on a vigorous bush" and are the true embodiments of evolutionarily successful groups.

The cone vs the pyramid

Gould argues that the conventional view of evolution, as illustrated by the cone of increasing diversity, is flawed. It is typically assumed that early life is restricted in form, and from this restriction of form follows diversification into the variety of animal life that currently exists. This cone can be visualized as an inverted Christmas tree, with a narrow base and numerous branches proliferating outward into the present day. Gould presents an alternative hypothesis, however, which states that the history of life is better described as "decimation followed by diversification within a few remaining stocks", represented as a pyramid with a wide base of anatomical disparity that becomes increasingly constrained by natural selection and extinction level events as time moves forward. This is evidenced by the fact that the fossils excavated from the Burgess Shale in British Columbia represent a paleo-ecosystem with much greater anatomical disparity than currently exists and that fewer phyla exist today compared to the Cambrian seas. Gould offers the view that life during the Cambrian explosion quickly proliferated into the diversity of forms seen today due to the availability of numerous ecological niches and was subsequently decimated by extinction level events throughout geological time. He also notes that the survival of groups following extinction events bears no relationship to traditional notions of Darwinian success in normal times. For example,

"Even if fishes hone their adaptations to peaks of aquatic perfection, they will all die if the pond dries up. But grubby old buster the lungfish, former laughing stock of the piscine priesthood, may pull through – and not because a bunion on his great-grandfather's fin warned his ancestors about an impending comet. Buster and his kin may prevail because a feature evolved long ago for a different use has fortuitously permitted survival during a sudden and unpredictable change in the rules. And if we are Buster's legacy, and the result of a thousand other similarly happy accidents, how can we possibly view our mentality as inevitable, or even probable?" – Stephen Jay Gould

Ultimately, Gould explains, both the false iconography of the march of progress and our allegiance to the cone of increasing diversity have led us astray in our thinking about trends in evolutionary biology.

Implications in the Origin of Life and Extraterrestrial Life Detection

The paper Alternative Pathways in Astrobiology: Reviewing and Synthesizing Contingency and Non-Biomolecular Origins of Terrestrial and Extraterrestrial Life extends Gould' contingency concept to the origins of lifeAbiogenesis, proposing that non-biomolecular chemistry may have played a significant role in the emergence of life on Earth. The authors argue that prebiotic environments likely contained a diverse array of non-biomolecular compounds that could have contributed to the formation of life. This challenges the traditional view that life must arise solely from biomolecules, such as proteins and nucleic acids, and suggests that life's origins may be more complex and varied.

The paper also addresses the "N = 1 problem," which refers to the limitation of basing all theories of life on a single example—life on Earth. This terrestrial bias could hinder the search for extraterrestrial life by assuming that alien life must conform to Earth-like biochemical frameworks. The authors propose a model that incorporates both deterministic and contingent processes, suggesting a spectrum of possibilities for how life could arise under different environmental conditions. This broader understanding of the origins of life, which includes both biomolecular and non-biomolecular pathways, has significant implications for astrobiology and the detection of extraterrestrial biosignatures.

Speculative evolution

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Speculative_evolution

A model of the hypothetical dinosauroid

Speculative evolution is a subgenre of science fiction and an artistic movement focused on hypothetical scenarios in the evolution of life, and a significant form of fictional biology. It is also known as speculative biology and it is referred to as speculative zoology in regards to hypothetical animals. Works incorporating speculative evolution may have entirely conceptual species that evolve on a planet other than Earth, or they may be an alternate history focused on an alternate evolution of terrestrial life. Speculative evolution is often considered hard science fiction because of its strong connection to and basis in science, particularly biology.

Speculative evolution is a long-standing trope within science fiction, often recognized as beginning as such with H. G. Wells's 1895 novel The Time Machine, which featured several imaginary future creatures. Although small-scale speculative faunas were a hallmark of science fiction throughout the 20th century, ideas were only rarely well-developed, with some exceptions such as Stanley Weinbaum's Planetary series, Edgar Rice Burroughs's Barsoom, a fictional rendition of Mars and its ecosystem published through novels from 1912 to 1941, and Gerolf Steiner's Rhinogradentia, a fictional order of mammals created in 1957.

The modern speculative evolution movement is generally agreed to have begun with the publication of Dougal Dixon's 1981 book After Man, which explored a fully realized future Earth with a complete ecosystem of over a hundred hypothetical animals. The success of After Man spawned several "sequels" by Dixon, focusing on different alternate and future scenarios. Dixon's work, like most similar works that came after them, were created with real biological principles in mind and were aimed at exploring real life processes, such as evolution and climate change, through the use of fictional examples.

Speculative evolution's possible use as an educational and scientific tool has been noted and discussed through the decades following the publication of After Man. Speculative evolution can be useful in exploring and showcasing patterns present in the present and in the past. By extrapolating past trends into the future, scientists can research and predict the most likely scenarios of how certain organisms and lineages could respond to ecological changes. In some cases, attributes and creatures first imagined within speculative evolution have since been discovered. A filter feeder anomalocarid was illustrated by artist John Meszaros in the 2013 book All Your Yesterdays by John Conway, C. M. Kosemen and Darren Naish. In the year following publication, a taxonomic study proved the existence of the filter feeding anomalocarid Tamisiocaris.

History

Early works

The Time Machine (1895) by H. G. Wells is seen by some as an early instance of speculative evolution and has been cited as an inspiration by later creators within the field.

Explorations of hypothetical worlds featuring future, alternate or alien lifeforms is a long-standing trope in science fiction. One of the earliest works usually recognized as representing one of speculative evolution is H. G. Wells's science fiction novel The Time Machine, published in 1895. The Time Machine, set over eight hundred thousand years in the future, features post-human descendants in the form of the beautiful but weak Eloi and the brutish Morlocks. Further into the future, the protagonist of the book finds large crab-monsters and huge butterflies. Science fiction authors who wrote after Wells often used fictional creatures in the same vein, but most such imaginary faunas were small and not very developed.

A four-armed "Green Martian" riding a "thoat" from Edgar Rice Burroughs's Barsoom, a fictional version of the planet Mars. Illustration by James Allen St. John (1920).

Edgar Rice Burroughs, who wrote in the early 20th century, can like Wells be considered an early speculative evolution author. Although his fictional ecosystems were still relatively small in scope, they were the settings of many of his novels and as such quite well-developed. In particular, Burroughs's Barsoom, a fictional version of the planet Mars which appeared in ten novels published from 1912 to 1941, featured a Martian ecosystem with a variety of alien creatures and several distinct Martian cultures and ethnic groups. Stanley Weinbaum's Planetary series also includes significantly conceptualized and developed alien life. Frederik Pohl wrote that before Weinbaum, science fiction's aliens "might be catmen, lizard-men, antmen, plantmen or rockmen; but they were, always and incurably, men. Weinbaum changed that. ... it was the difference in orientation – in drives, goals and thought processes – that made the Weinbaum-type alien so fresh and rewarding in science fiction in the mid-thirties."

A mock taxidermy of a rhinograde, using its nasorium to catch fish. Rhinogrades, created by Gerolf Steiner in 1957, are one of the earliest concrete examples of speculative zoology.

In 1930, Olaf Stapledon published a "future history", Last and First Men: A Story of the Near and Far Future, describing the history of humanity from the present onwards, across two billion years and eighteen human species, of which Homo sapiens is the first. Besides conventional environment-driven evolution -during which offshoots of humanity experienced both elevated and the total loss of sentience - the book anticipates the science of genetic engineering, and is an early instance of the fictional group mind idea. Published in 1957, German zoologist Gerolf Steiner's book Bau und Leben der Rhinogradentia (translated into English as The Snouters: The Form and Life of the Rhinogrades) described the fictional evolution, biology and behavior of an imaginary order of mammals, the Rhinogradentia or "rhinogrades". The Rhinogrades are characterized by a nose-like feature called a "nasorium", the form and function of which vary significantly between species, akin to Darwin's finches and their beak specialization. This diverse group of fictional animals inhabits a series of islands in which they have gradually evolved, radiating into most ecological niches. Satirical papers have been published continuing Steiner's imagined world. Although the work does feature an entire speculative ecosystem, its impact is dwarfed by the later works due to its limited scope, only exploring the life of an island archipelago.

In 1976, the Italian author and illustrator Leo Lionni published Parallel Botany, a "field guide to imaginary plants", presented with academic-style mentions of genuine people and places. Parallel Botany has been compared to the 1972 book Invisible Cities by Italo Calvino, in which Marco Polo in a dialogue with Kublai Khan describes 55 cities, which, like Lionni's "parallel" plants, are "only as real as the mind's ability to conceptualize them".

Movement

Author Dougal Dixon with a model of a "Strida", one of the creatures featured in his 2010 book Greenworld.

One of the significant "founding" works of speculative evolution is After Man by Dougal Dixon, published in 1981. To this day, After Man is recognized as the first truly large-scale speculative evolution project involving a whole world and a vast array of species. Furthering its significance is the fact that the book was made very accessible by being published by mainstream publishers and being fully illustrated with color images. As such, After Man is often seen as having firmly established the idea of creating entire speculative worlds. Through the decades following After Man's publication, Dixon remained one of the sole authors of speculative evolution, publishing two more books in the same vein as After Man; The New Dinosaurs in 1988 and Man After Man in 1990. Dixon cited The Time Machine as his primary inspiration, being unaware of Steiner's work, and devised After Man as a popular-level book on the processes of evolution that instead of using the past to tell the story projected the processes into the future. A central idea of After Man, besides a wave of extinction following humans, is convergent evolution as new species bear a close resemblance to their unrelated predecessors.

When designing the various animals of the book, Dixon looked at the different types of biomes on the planet and what adaptations animals living there have, designing new animals descended from modern day ones with the same set of adaptations. The success of After Man inspired Dixon to continue writing books that explained factual scientific processes through fictional examples. The New Dinosaurs was in essence a book about zoogeography, something the general public would be unfamiliar with, using a world in which the non-avian dinosaurs had not gone extinct. Man After Man, explored climate change over the course of the next few million years by showcasing its effects through the eyes of future human descendants.

Today, many artists and writers work on speculative evolution projects online, often in the same vein as Dixon's works. Speculative evolution continues to endure a somewhat mainstream presence through films and TV shows featuring hypothetical and imaginary creatures, such as The Future is Wild (2002), Primeval (2007–2011), Avatar (2009), Terra Nova (2011), and Alien Worlds (2020). The modern explosion of speculative evolution has been termed by British paleontologist Darren Naish as the "Speculative Zoology Movement".

As an educational and scientific tool

Reconstruction of Tamisiocaris (top), an anomalocarid from the Cambrian which was discovered to have been a filter-feeder in 2014. A hypothetical filter-feeding anomalocarid was featured in the book All Your Yesterdays (2013).

Although primarily characterized as entertainment, speculative evolution can be used as educational tool to explain and illustrate real natural processes through using fictional and imaginary examples. The worlds created are often built on ecological and biological principles inferred from the real evolutionary history of life on Earth and readers can learn from them as such. For example, all of Dixon's speculative works are aimed at exploring real processes, with After Man exploring evolution, The New Dinosaurs zoogeography and both Man After Man and Greenworld (2010) exploring climate change, offering an environmental message.

In some cases, speculative evolution artists have successfully predicted the existence of organisms that were later discovered to resemble something real. Many of the animals featured in Dixon's After Man are still considered plausible ideas, with some of them (such as specialized rodents and semi-aquatic primates) being reinforced with recent biology studies. A creature dubbed "Ceticaris", conceived by artist John Meszaros as a filter-feeding anomalocarid, was published in the 2013 book All Your Yesterdays, and in 2014, the actual Cambrian anomalocarid Tamisiocaris was discovered to have been a filter-feeder. In honor of Meszaros's prediction, Tamisiocaris was included in a new clade named the Cetiocaridae.

Dougal Dixon's The New Dinosaurs was heavily influenced by paleontological ideas developing during its time, such as the ongoing dinosaur renaissance, and as such many of the dinosaurs in the book are energetic and active creatures rather than sluggish and lumbering. Dixon extrapolated on the ideas of paleontologists such as Robert Bakker and Gregory S. Paul when creating his creatures and also used patterns seen in the actual evolutionary history of the dinosaurs and pushing them to an extreme. Perhaps because of this, many of the animals in the book are similar to actual Mesozoic animals that were later discovered. Many of the dinosaurs in it are feathered, something not widely accepted at the time of its publication but seen as likely today. Similarly, After Man in 1981 represents a sort of time capsule of geological thought before global warming was fully discerned, but Dixon also portrays a sixth mass extinction or Anthropocene before it was commonplace to do so.

Hypothetical restoration of Dromaeosauroides bornholmensis, which is known from two teeth. Its appearance is inferred from related genera.

Speculative reconstruction of Sinopliosaurus fusuiensis with generalized spinosaurid morphology, and unique coloration pattern.

Speculative evolution can be useful in exploring and showcasing patterns present in the present and in the past, and there is a useful aspect to hypothesizing on the form of future and alien life. By extrapolating past trends into the future, scientists could research and predict the most likely scenarios of how certain organisms and lineages could respond to ecological changes. As such, speculative evolution facilitates authors and artists to develop realistic hypotheses of the future. In some scientific fields, speculation is essential in understanding what is being studied. Paleontologists apply their own understanding of natural processes and biology to understand the appearances and lifestyles of extinct organisms that are discovered, varying in how far their speculation goes. For instance, All Yesterdays and its sequel All Your Yesterdays (2017) explores highly speculative renditions of real (and in some cases hypothetical) prehistoric animals that do not explicitly contradict any of the recovered fossil material. The speculation undertaken for All Yesterdays and its sequel has been compared to that of Dixon's speculative evolution works, though its objective was to challenge modern conservative perceptions and ideas of how dinosaurs and other prehistoric creatures lived, rather than designing whole new ecosystems. The books have inspired a modern artistic movement of artists going beyond conventional paleoart tropes, expanding into increasingly speculative renditions of prehistoric life.

Additionally, the evolutionary history of fictional organisms has been used as a tool in biology education. Caminalcules, named after Joseph H. Camin, are a group of animal-like lifeforms, consisting of 77 purported extant and fossil species that were invented as a tool for understanding phylogenetics. The classification of Caminalcules, as well as other fictional creatures such as dragons and aliens, have been used as analogies to teach concepts in evolution and systematics.

Speculative evolution is sometimes presented in museum exhibitions. For instance, both After Man and The Future is Wild has been presented in exhibition form, educating museum visitors on the principles of biology and evolution through using their own fictional future creatures.

Subsets

Extraterrestrial life

See also: Astrobiology, Xenobiology, and Extraterrestrials in fiction

A popular subset of speculative evolution is the exploration of possible realistic extraterrestrial life and ecosystems. Speculative evolution writings focusing on extraterrestrial life, like the blog Furahan Biology, use realistic scientific principles to describe the biomechanics of hypothetical alien life. Although commonly identified with terms such as "astrobiology", "xenobiology" or "exobiology", these terms designate actual scientific fields largely unrelated to speculative evolution. Though 20th century work in exobiology sometimes formulated "audacious" ideas about extraterrestrial forms of life. Astrophysicists Carl Sagan and Edwin Salpeter speculated that a "hunters, floaters and sinkers" ecosystem could populate the atmospheres of gas giant planets like Jupiter, and scientifically described it in a 1976 paper.

In extraterrestrial-focused speculative biology, lifeforms are often designed with the intention to populate planets wildly different from Earth, and in such cases concerns like chemistry, astronomy and the laws of physics become just as important to consider as the usual biological principles. Very exotic environments of physical extremes may be explored in such scenarios. For example, Robert Forward's 1980 Dragon's Egg develops a tale of life on a neutron star, and the resulting high-gravity, high-energy environment with an atmosphere of iron vapor and mountains 5-100 millimeters high. Once the star cools down and stable chemistry develops, life evolves extremely quickly, and Forward imagines a civilization of "cheela" that lives a million times faster than humans.

In some cases, artists and writers exploring possible alien life conjure similar ideas independent of each other, often attributed to studying the same biological processes and ideas. Such occasions can be called "convergent speculation", similar to the scientific idea of convergent evolution.

Tytge Sea Leviathan (the creature in the center), from the sci-fi franchise Infinity Horizon.

Perhaps the most famous speculative work on a hypothetical alien ecosystem is Wayne Barlowe's 1990 book Expedition, which explores the fictional exoplanet Darwin IV. Expedition was written as a report of a 24th-century expedition that had been led to the planet by a team composed of both humans and intelligent aliens and used paintings and descriptive texts to create and describe a fully realized extraterrestrial ecosystem. Barlowe later served as an executive producer of a TV adaptation of the book, Alien Planet (2005) where exploration of Darwin IV is instead carried out by robotic probes and the segments detailing the ecosystems of the planet are intercut with interviews with scientists, such as Michio Kaku, Jack Horner and James B. Garvin.

Other examples of speculative evolution focused on extraterrestrial life include Dougal Dixon's 2010 book Greenworld, TV programmes such as 1997 the BBC2/Discovery Channel special Natural History of an Alien and the 2005 Channel 4/National Geographic programme Extraterrestrial as well as a variety of personal web-based artistic projects, such as C. M. Kosemen's "Snaiad" and Gert van Dijk's "Furaha", envisioning the biosphere of entire alien worlds.

Through science fiction, the speculative biology of extraterrestrial organisms has a strong presence in popular culture. The eponymous monster of Alien (1979), particularly its life cycle from egg to parasitoid larva to 'Xenomorph', is thought to be based on the real habits of parasitoid wasps in biology. Further, H. R. Giger's design of the Alien incorporated the features of insects, echinoderms and fossil crinoids, while concept artist John Cobb suggested acid blood as a biological defense mechanism. James Cameron's 2009 film Avatar constructed a fictional biosphere full of original, speculative alien species; a team of experts ensured that the lifeforms were scientifically plausible. The creatures of the movie took inspiration from Earth species as diverse as pterosaurs, microraptors, great white sharks, and panthers, and combined their traits to create an alien world. Darren Naish praised the creature design of 2022's Avatar: The Way of Water as well, admitting suspension of disbelief on the humanoid Na'vi protagonists. He notes the other creatures, aliens and their anatomies and lifestyles are inspired by evolution and ecology to a significant degree, with probable inspirations such as mycorrhizal fungi, marine reptiles, and simian evolution. According to Naish, "the series will be a mainstay in discussions about creature design and speculative biology for some time yet."

Alternative evolution

Speculative zoology can examine sometimes overlooked prehistoric animals in an evolutionary context. The Speculative Dinosaur Project focused as much on mammals, squamates, and crocodylomorphs as on dinosaurs. Pictured are metatherian marsupials that have converged on our universes' mustelids.

Similar to alternate history, alternative evolution is the exploration of possible alternate scenarios that could have played out in the Earth's past to give rise to alternate lifeforms and ecosystems, popularly the survival of non-avian dinosaurs to the present day. As humanity is often not a part of the worlds envisioned through alternative evolution, it has sometimes been characterized as non-anthropocentric.

Although dinosaurs surviving to the age of humans has been adapted as a plot point in numerous science fiction stories since at least 1912, beginning with Arthur Conan Doyle's The Lost World, the idea of exploring the fully fledged alternate ecosystems that would develop in such a scenario truly began with the publication of Dixon's The New Dinosaurs in 1988, in which dinosaurs were not some lone stragglers of known species that had survived more or less unchanged for the last 66 million years, but diverse animals that had continued to evolve beyond the Cretaceous. In the vein of Dixon's The New Dinosaurs imagination, a now largely defunct, but creatively significant collaborative online project the Speculative Dinosaur Project followed in the same zoological worldbuilding tradition.

Since 1988, alternative evolution has sometimes been applied in popular culture. The creatures in the 2005 film King Kong were fictitious descendants of real animals, with Skull Island being inhabited by dinosaurs and other prehistoric fauna. Inspired by Dougal Dixon's works, the designers imagined what 65 million years or more of isolated evolution might have done to dinosaurs.^[50] Concept art for the film was published in the book The World of Kong: A Natural History of Skull Island (2005), which explored the world of the film from a biological perspective, envisioning Skull Island as a surviving fragment of ancient Gondwana. Prehistoric creatures on a declining, eroding island had evolved into "a menagerie of nightmares".

A hypothetical natural history of dragons is a popular subject of speculative zoology, being explored in works such as Peter Dickinson's The Flight of Dragons (1979), the 2004 mockumentary The Last Dragon and the Dragonology series of books.

Future evolution

The evolution of organisms in the Earth's future is a popular subset of speculative evolution. A relatively common theme in future evolution is civilizational collapse and/or humans becoming extinct due to an anthropogenic extinction event caused by environmental degradation. After such a mass extinction event, the remaining fauna and flora evolve into a variety of new forms. Although the foundations of this subset were laid by Wells's The Time Machine already in 1895, it is generally agreed that it was definitively established by Dixon's After Man in 1981, which explored a fully realized future ecosystem set 50 million years from the present. Dixon's third work on speculative evolution, Man After Man (1990) is also an example of future evolution, this time exploring an imagined future evolutionary path of humanity.

Peter Ward's Future Evolution (2001) makes a scientifically accurate approach to the prediction of patterns of evolution in the future. Ward compares his predictions with those of Dixon and Wells. He tries to understand the mechanism of mass extinctions and the principles of recovery of ecosystems. A key point is that "champion supertaxa" who diversify and speciate at a greater rate, will inherit the world after mass extinctions. Ward quotes the paleontologist Simon Conway Morris, who points out that the fantastical or even whimsical creatures devised by Dougal Dixon, echo nature's tendency to converge on the same body plans. While Ward calls Dixon's visions "semi-whimsical" and compares them to Wells' initial visions in The Time Machine, he nonetheless continues the use of analogous evolution, which is a larger trend in speculative zoology.

Future evolution has also been explored on TV, with the mockumentary series The Future is Wild^[56] in 2002, for which Dixon was a consultant (and author of the companion book), and the series Primeval (2007–2011), a drama series in which imagined future animals occasionally appeared. Ideas of future evolution are also frequently explored in science fiction novels, such as in Kurt Vonnegut's 1985 science fiction novel Galápagos, which imagines the evolution of a small surviving group of humans into a sea lion-like species. Stephen Baxter's 2002 science fiction novel Evolution follows 565 million years of human evolution, from shrewlike mammals 65 million years in the past to the ultimate fate of humanity (and its descendants, both biological and non-biological) 500 million years in the future. C. M. Kosemen's 2008 All Tomorrows similarly explores the future evolution of humanity. Speculative biology and the future evolution of the human species are significant in bio art.

Seed worlds

Seed worlds, or seeded worlds, are another popular subset of the genre. It involves a terraformed planet or a habitable, yet uninhabited planet being "seeded" by already existing species of animals, plants and fungi, which will speciate in order to fill the different niches by adaptive radiation. The focus can be on one or multiple species, but usually more taxa are present on the project's planet, that won't be covered in as much detail.

One of the most well-known works in this category is Serina: A Natural History of the World of Birds by Dylan Bajda, in which the focal species is the domestic canary, Serinus canaria domestica, who is the progenitor of all other bird species that come later. A minor species that later becomes more relevant is the guppy (Poecilia), whose descendants become terrestrial tripods and compete against the birds after a severe mass extinction which killed 99% of all species on the moon. Another relevant seed world, Batrachiterra, involves various species of frogs seeded by humans on the fictional planet Heqet, originally for the purpose of studying batrachotoxin.