Imagine
yourself inside a completely empty void. When I say empty, not only
do I mean that there are no objects, no mass or energy, but also that
there is neither time nor space. There is only compete and utter
nothingness, everywhere and everywhen. Bear with me, because I am
really quite serious about this: close your eyes, screen out as much
of the world around you as you can, and really try to place yourself
in just this situation.
Difficult,
isn’t it? In fact, I’ll bet that you find it utterly impossible.
There is a good reason for this. It is impossible. It is
impossible because the very act of imagining a void places you
inside the void, whereupon it is a void no longer. The moment
you are there you are mass / energy taking up volume, and so there
are these phenomenom, and well as space. Events are happening (even
if just the beating of your heart), so time exists as well. Mass,
energy, space, and time are inexorably linked together. You cannot
have one without the other three. As you are mass / energy, taking
up space / time, the very act of trying to imagine an empty void ends
its existence.
This
is, admittedly, more of a philosophical argument than anything
derived from physics, and I suspect that any physicists reading it
are doing so with a jaundiced eye. It certainly isn’t a rigorous
argument, but I think it is a good place to start wondering about the
past because, if a lot of modern physics is correct (there are
dissenting views), our entire universe has a beginning, before which
there really was nothing, mass, energy, space, or time. Perhaps
there were no physical laws either, as difficult as it is to see how
the very concept can make any sense.
That
beginning has a name, a name that ironically has found its way into
common usage because its author, Fred Hoyle, intended it to be more
of an insult to discredit it than anything else. It is, admittedly,
a rather silly name for a scientific theory, compared to, say,
General Relativity or Quantum Mechanics or Evolution by Natural
Selection. It also has the flaw of not being a particularly accurate
way of describing the beginning of the universe; the phrase “Big
Bang” implies an explosion of matter and energy within a
pre-existing space and time, rather than the initial expansion of
space-time as well the matter and energy which occupies it.
In
calling it “the initial expansion” I am not being much more
accurate, however. There is no point in the evolution of the
universe where can point to and say, “The Big Bang ends here;
everything after this point is just ordinary expansion.” The truth
is, the Big Bang is not a theory of how the universe got started; it
is a theory of how it has evolved and still is evolving and will
evolve that reaches right back to its very beginnings, back to time
zero, assuming that there was a time zero and whether that term has
any meaning. It is also not a theory of how or why that expansion
began; there is a lot of scientific speculation about what might have
been happening before time zero which lead to the Big Bang,
but (almost) none of that speculation describes or modifies the Bang
itself. Lastly, an important perspective to maintain is that we are
not mere observers of but are living amidst the Big Bang, yes, right
now even as we go about our busy little lives it is happening right
here and everywhere else in the universe, an event which probably has
many, many trillions of years to go or more, assuming that it will
not continue forever, which at this point in our understanding of it
is not at all clear.
And
having said all that, I must now confess that I’m still not
describing it with sufficient precision. I speak as though it is
iron-clad scientific fact that the universe has a beginning. We do
not understand the laws of physics well enough, however, to make so
powerful a claim. All we can do is: ( a) observe now that the
universe is expanding; (b) imagine reversing time so that the
universe is contracting; and (c) extrapolate the time-reversed
contraction until we inevitably encounter a situation in which all
matter, energy, space, and time in the universe are compressed into a
single point, a so-called “singularity”, at which moment our
extrapolation comes to an end by logical necessity – by
mathematical definition, nothing can by smaller than a single point.
This is where we run into trouble, however. We cannot follow (c) to
its logical conclusion with certainty because nobody knows what the
laws of physics are beyond a certain degree of compression and
temperature; as the universe gets smaller, hotter, and denser the set
of rules of how matter-energy-space-time behave under such conditions
are less and less well understood. When you get to the point where
you don’t know what the rules are anymore, you simply can’t
extrapolate any further. Thus, (c) is really just a conjecture; we
cannot say whether there was a beginning or not. It is an important
point to keep in mind because you will routinely encounter statements
such as “the universe is 13.73 ± 0.12 billion years old” which
assume (c) is true. Actually, such statements are really
conveniences, because they allow us to apply time markers to various
stages of the universe’s evolution.
The
result of all this is that we are face to face with a dilemma.
Curiosity and wonder cause us to yearn for answers that our current
state of knowledge can’t gratify. Worse, we may never have
gratification, because we are bumping up against the limits of what
technology and even the laws of physics themselves can help us
answer. The most powerful telescopes in the world can’t see all
the way back to the beginning, if there is one, of the universe
(remember that in looking at objects further and further away in
space, the time it takes for the light to reach us means we are
looking further and further into the past). The most powerful
particle accelerators we have built can’t come close to reproducing
the earliest fraction of a second of the universe’s existence.
Even the theoretical / mathematical approach, through it has paid its
share of dividends in nuclear physics (it is how Murray Gell-Mann
predicted the existence of quarks for example), might not get us
there. It is as though we keep creeping closer and closer to our
goal, yes, but we can never actually get there, however patient and
persistent we are.
Still,
the ability to trace back the evolution of the cosmos back 13.73 ±
0.12 billion years is a remarkable feat, one of the most remarkable
in the history of science, even if we struggle to squeeze the
previous trillionth of a second or so out of our cosmic clock. We
are talking of a time long before our solar system existed, indeed
long before there were any stars or planets or galaxies or kinds of
structures we see in the universe today. What we can say is,
however, is that the primordial seeds of all these structures,
including ourselves, must have somehow existed in the super-hot,
super-dense soup of quarks, gluons, electrons, neutrinos, photons,
and what else have you which constituted the universe – or at least
our universe – at that time. How those seeds of so long ago became
the reality we experience around and within us today is a fantastic
tale, only a fraction of which has been worked out to any detail.
* * *
So
much for ultimate beginnings. Let us jump forward to more mundane
affairs. I suspect that most people, if asked what the early Earth
was like, would probably think of dinosaurs roaming its landscapes,
and their reptilian cousins filling the air and the oceans. Some
would even think of “cave men” (not all early humans lived in
caves, of course) hunting wooly mammoths and other prehistoric
creatures. I hope these statements don’t sound condescending,
because they aren’t meant to be. Rather, they are meant as
cautions regarding our sense of time, particularly our sense of
so-called “deep” time, that time scale in which biological,
geological, and cosmic evolution reveal their full and wondrous
workings. Time of this magnitude overwhelms our imaginations. We
simply cannot, however hard we try, conceive of a million years.
Even a thousand years – a thousandth of that million, and the
approximate distance between Middle Age Europe and today – defies
our intuitive sense of time. Actually, as Richard Dawkins points out
in The Blind Watchmaker this is not only unsurprising but
exactly what we should expect: our intuitive sense of time is the
result of the way our brains are wired, and the genes that have wired
them have been selected by evolution to give us good, gut-level
grasps only of events that take from significant fractions of a
second to decades to happen. Any event much shorter than a second or
longer than a hundred years or so fall outside of the range of our
intuitive time sense, and we shall have a hard time conceiving them.
This is why a thousand, a million, a billion, and a trillion years
all feel pretty much the same to us, although there is a thousand
fold difference between each pair, thousand to a million and so on.
Again, as Dawkins has noted, this is probably one of the main reasons
for most of the people who reject evolution; they simply cannot
imagine the time scales involved, and therefore seriously
underestimate what is and isn’t probable during them.
* * *
Typically,
this is where most authors of books dealing with deep time try to
come up with some kind of scale which makes these times more
intuitively manageable. The scale often relates time to distance; so
that for example the age of the universe may be represented by a line
stretching from Los Angeles to New York City, or something like that.
The trick in using this analogy is in avoiding scales which already
defy readers’ imaginations at one or both ends; I, for one, am not
sure I can intuitively grasp the three thousand mile LA-NYC distance
effectively. This of course is because our brains have also been
selected to have good, gut-level grasps of distances as well as
times, and three thousand miles almost certainly falls well outside
of that range as well.
On
the other hand, too small a scale, while allowing us to grasp its
full length, can be too short to let us sense the fractions of that
scale we wish to deal with. For an example of this, a universe
timeline only a mile long would put our human lifespans in a fraction
of a thousandth of an inch range; most of us, I suspect, find that
just as hard to fathom as the LA-NYC distance.
What
we want is a scale that is neither too large in its full range, nor
too short in the fractions of that range we wish to work with. The
scale I’ve chosen to work with seems pretty workable to me. What I
propose to do is simply scale all times down by a factor of a
billion. I am also leaving it as a time scale, rather than
converting it to distance as many people do. In this scale,
therefore, the universe is about 13.7 years old. As human beings in
the real universe generally live between two and three billion
seconds (there are 31,556,926 seconds in a year, or a billion seconds
in 31.688765 years), our average lifetimes are reduced to between two
and three seconds using this scale. Events thus appear pretty
manageable at both ends of the continuum, or at least they do so for
me.
If
we take this moment, now, December 18, 2008 as I write these words,
as the end point of our scale, then the universe “began” (recall
my cautionary statements earlier about the beginning of the universe)
in early April of 1995. Relating that to my own life, I was
thirty-eight years old then compared to fifty-two today, my 21½ year
old daughter was nearing her ninth birthday, and my 14½ year old son
was only a baby, not yet taking his first steps yet. President Bill
Clinton was in his first term, it was about four years after the fall
of the Soviet Union, and the terrorist attacks of 9/11/2001 were
still over six years in the future. I can relate a lot of other
events of that time in my life, and no doubt so can the reader
(unless you are a teenager or younger).
There
are numerous ways we can proceed here. For example, the expanding
universe became transparent – that is, it cooled off enough for the
first atoms to form, allowing electromagnetic radiation to decouple
from matter and stream freely through space in what is now called the
cosmic background radiation – about 3½ hours later on that April
day, and the first galaxies that we can detect with our most powerful
telescopes would have formed about a year later, in early 1996. We
don’t know with certainty when our own Milky Way galaxy condensed
from intergalactic gas and dust, but it must have been well before
the creation of our solar system 4½ years ago, in June, 2004. This
is because our sun is a so-called “first generation star”, one
heavily composed from the ejecta of earlier, massive stars which
exploded in supernovae and seeded interstellar space with all the
elements up to uranium (thereby allowing the sun to have terrestrial
planets like Earth).
If
we restrict our timeline to events that happened on Earth, then the
first fossil traces of simple “prokaryotic” (bacteria or archea)
cells appear as either rare microfossils or biochemical traces
between 4 and 3½ years ago, although recent hypotheses about early
Earth could mean life began sooner. Eukaryotic cells, which are
larger, more complex cells which comprise all animals, plants, and
fungi today, evolved from prokaryotic cells at around somewhat more
that half that distance from the present, or around two years ago,
and the first, unequivocal albeit very simple multicellular organisms
appear, along with sex, approximately a little over a year ago,
although this is uncertain because such organisms contained no hard
parts (shells, skeletons, etc.) and would have left few fossils.
At
this point, let me backtrack a little. Earlier I mentioned how most
people probably think of dinosaurs, or even early humans aka cave
men, as living on the early Earth. We begin to see now just how
potent a testament to our poor intuitive grasp of time that is. So
far I’ve covered about three-quarters of the Earth’s history, and
over ninety percent of the universe’s and nowhere have dinosaurs or
early humans been mentioned. In fact, we still have some
considerable chronology to cover before we get there.
Scientists,
as they so often do whatever their field of specialty, have divided
Earth’s history into a number of time slices called eons
(technically, an eon is a billion years, so it is used only as a
rough approximation here). These are, from oldest to most recent,
the Hadean eon, covering the time between Earth’s formation and 3.8
billion years ago; the Archeon eon, when life unequivocally first
began, lasting from 3.8 billion years ago to 2.5 billion years ago;
the Proterozoic eon, from 2.5 billion to 542 million years ago,
covering the evolution of eukaryotic cells and the first
multi-cellular organisms; and the Phanerozoic eon, which takes us
from the end of the Proterozoic to the present. Each of these eons
begins and ends with some important developments in the evolution of
life on our planet: the first appearance of life, the first
eukaryotes, and so on (the burying of the first multicellular
organisms and sex in the middle of the Proterozoic seems to be the
major exception to this schema).
On
our 109 : 1 scaled down history of the universe, the
Proterozoic Eon lasts from two and a half years to six and a half
months ago. The last three and a half months (starting from ten
months or 850 million years BP) of this epoch are of immense
importance to biologists, geologists, and paleontologists because it
includes a series of events which may have given rise to the world we
know today, teaming with highly complex, diversified multicellular
organisms – such as ourselves – which dominate almost everywhere
we look.
The
first such series are a sequence of serious ice ages, lasting from
850-800 million (ten months) to around 630 million (seven and a half
months) ago. There is considerable controversy over just how severe
these ice ages were; the phrase “Snowball Earth” has been coined
by those scientists who believe virtually the entire planet was
entirely frozen over, right down to the tropics, while critics of
this admittedly dramatic picture believe the planet came nowhere
close to such extremes – the phrase “Slushball Earth” has been
used by a number of said critics. Other scientists dispute the
timing as well as the extents of the ice expanse. It appears also
that these ice ages were interspersed with periods of unusual warmth.
This is because greenhouse gasses such as carbon dioxide (CO2),
methane (CH4), and sulfur dioxide (SO2) belched
from active volcanoes would have built up in the atmosphere during
the ice ages, yet the extremely cold, dry atmosphere would have been
unable to precipitate them out; when rising temperatures caused by
this CO2 / CH4 / SO2 driven
greenhouse effect finally did melt the ice there might have been
temporary “Hot House Earths” until the excess gas was finally
removed.
Whatever
the exact scenario, these large climactic swings must have had
devastating effects on Earth’s biota of the time. We may be quite
fortunate that all life was not completely and permanently
extinguished. Certainly, many species would not have been able to
cope with the prevailing conditions and did become extinct, while
those that did survive probably stumbled upon various adaptations
which natural selection would have favored. In short, the Snowball /
Hot House Earth period probably was a crucible in which evolutionary
pressures would have been much greater than at almost any other
period in Earth’s history – a biological ice house / smelter from
which those who emerged would have many improvements over those who
went into it. This is a somewhat speculative statement, but it is a
reasonable one. The fact that the first truly complex multicellular
organisms appear in the fossil record only a few million years after
the end of the Proterozoic ice ages (about two weeks in our timeline)
– the so-called Ediacaran fauna of soft-bodies animals – lends
support to it.
Let’s
stop, catch our breaths, and take stock of where we are and how far
we’ve come. In mentioning the first appearance of the Ediacaran
fauna we are standing somewhere between 600 and 580 million years BP.
That is about seven months BP in our scaled-down timeline. Given
that Earth first condenses from the solar nebula 4½ years, or 54
months, ago on this timeline, we have already covered 1
47/54 = 87% of our planet’s history (or, that the universe begins
13.73 years or 165 months ago, 1
157/165 = 96% of its history). By this reckoning, even Ediacaran
animals are the new neighbors on the block, although they don’t
hang around for long: by six and a half months ago or 86.5% of
Earth’s history they have vanished forever.
Dinosaurs?
Early humans? Not even close yet.
* * *
As
a child, probably my second most favorite activity, after star
gazing, was fossil hunting. There was a small creek which ran very
close to the house I grew up in, and from time to time I would spend
a few hours at one of the sandy banks and rock outcroppings along the
beach, turning over stone after stone in hopes of find one with the
outlines of some ancient life form on it (those that didn’t I would
practice stone-skipping with if I could find stones flat enough).
Mind you, I didn’t have a fossil collection, not in any kind of
formal sense; I just kept whatever I found of interest in the various
places I squirreled stuff, taking them out occasionally to look at
and wonder about. I wouldn’t even hazard a guess as to where any
of them are now; most of them are probably back in the ground I
liberated them from, hopefully to be re-unearthed by future
generations.
I
don’t remember if the connection between the two pastimes, star
gazing and fossil hunting, ever occurred to me back then, but it is
quite obvious now. Both are attempts to peer into the past, to learn
about what was, not just a few years or even centuries but thousands
and even millions of years ago. There is something about the
vastness of deep time, like deep space, that is paradoxically both
very intimate and infinitely far away at the same time; it is, I
sometimes suspect, rather like being with someone we have known and
loved for a long time, but still realize we don’t know very well.
I
didn’t become either a paleontologist or an astronomer because, as
I have already confessed to, I’m too much of a scientific
dilettante, and too lazy I must also add, to put in the time and
effort or endure the tedium that being a professional in these fields
requires. So I don’t hunt for fossils anymore; I would no doubt
chuck them anywhere convenient, something that I now know but my
child self didn’t is akin to blasphemy. Let someone who will treat
them with appropriate respect discover them.
The
very idea of a fossil is a fantastic one, one that makes us shake our
heads in near disbelieving wonderment when we think about it. Think
of all the many quadrillions of living things that must have lived
and died on this planet. Despite such numbers, that any of them
should have had their hard parts (and sometimes soft parts)
mineralized into or left impressions in rock, and then be exposed to
human eyes millions of years later by weathering or geological
processes is, I think, amazing enough. But to in addition be
discovered by scientists who study them and use them to piece
together so much of the story of life on Earth as they have, is one
of the most remarkable accomplishments of science. For me it is
certainly well up there with the discovery of the laws of physics or
the elucidation of the details of biological heredity.
I
have paused in our cosmological timeline to talk about fossils
because for most intents and purposes this is where life really
begins. True, we do have fossils and fossil traces and other
biochemical markers in rocks older than 540-550 million years ago
(six and a half months on our timeline) but they are much rarer and
difficult to find because before this time few if any living things
possessed hard parts to leave behind when they died. This is the
start of the Cambrian period of our current era of life, the
Phanarozoic eon; this is the beginning of many animals possessing
shells or exo or endoskeletons which can mineralize after the
animal’s death or leave clear impressions in rock.
One
of the most fascinating – and scientifically challenging – things
about the Cambrian is how quickly this evolution and radiation of
creatures with hard parts occurs. Quickly on the geologic scale that
is; within a few tens of millions of years at most, or about a week
on our timeline, most of the major phyla (large groups of life, such
as vertebrates, arthropods, mollusks, etc.) have made their first
appearance in the fossil record. This period is even often termed
the Cambrian Explosion in recognition of its extreme rapidity,
although of course by human time standards it is still an immense
stretch of time, well, well beyond anything we can intuitively grasp.
It is as though the speed of evolution had been stepped up by an
order of magnitude or more for a good several tens of millions of
years or more. Scientists are still debating the cause of the
Cambrian Explosion, with some arguing that it really wasn’t really
an explosion at all but more of an artifact of the fossil record.
Time, if the pun can be pardoned, will tell. Either way, by some 500
million years or six timeline months or so ago almost all the major
animal body plans had been laid down and are ready for evolution to
elaborate and sculpt them into all the animal species found on Earth
today. An Earth that is about ninety percent of its present age, or
ninety-seven percent of the universe’s. And yes, we still have a
way to go before the first dinosaurs appear.
* * *
I
hope that by now you have started to get at least some feel for the
overwhelming immenseness of geologic, or more, of cosmic time.
During the period we have been covering so far, somewhere around a
couple of billion stars have been born, lived, and perished in our
Milky Way galaxy alone, and probably over a billion trillion in the
universe as a whole, a universe that has expanded from something much
smaller than an atom into an empire of galaxies, galactic clusters,
and galactic superclusters spreading over tens of billions of
light-years of spacetime. It all begs the natural question: have
any other civilizations preceding our own come into existence, and
perhaps went extinct, in all this enormity? Have any other minds
even puzzled over the questions we puzzle over now? It seems so odd
that the answer to this question is that we simply don’t know, and
may never know, however hard we search for it.
Based
on what we do know, our own existence was not even remotely close to
guaranteed at the end of the Cambrian, that half billion years / six
months ago. The fact is, even today, almost one hundred and fifty
years after the publication of the Origin of Species, we still
don’t know in any detail why some species lines survive while
others go extinct. We comprehend in broad brushstrokes about natural
selection and adaptive complexity and the competition for survival
and reproduction, yes; but we can’t point to any fossil in a
geologic strata and say for sure why it does or doesn’t exist in
the strata directly above or below it. There are general trends in
the fossil record that can be followed and so offer hope for better
understanding: an increasing number and diversification of species
and genera (punctuated by mass extinction events which we will come
to shortly); long term increases and decreases in the sizes of the
largest species; a general overall though not perfectly steady trend
towards increasing complexity and intelligence, assuming that cranium
versus overall body size is a good indicator of intelligence. We
can, if we squint our eyes tight enough and apply a liberal enough
dose of imagination and wishful thinking, perceive humans or
something like humans eventually rising from the fray. But we don’t
really know, whatever we privately, or publically, believe.
The
biggest monkey wrenches thrown into our hopeful perception of
progress from the Cambrian onward are the mass extinction events that
pock-mark the fossil record like shotgun blasts from a drunkard.
There are five recognized major ones during Phanerozoic eon (the
criterion for being major is that over fifty percent of existing
animal species were killed off), along with a myriad of smaller
events. The most important effect of these events is how they often
drove many hitherto successful species, genera, families, and even
entire orders into extinction, essentially “clearing the slate”
so that new kinds of animals could rise to prominence. Life on Earth
today would be much different had they not occurred. We, for one,
would certainly not be here to discuss them. With almost equal
certainty neither would the dinosaurs, whose rise and fall appear to
be the result of three of the five major events.
Let’s
return to our timeline, where we left off six months, or 500 million
years ago, bearing in mind again that we are almost at ninety percent
of Earth’s present age. If we fast-forward to half that distance
in the past, three months / 250 million years ago, we find that our
Cambrian beginnings have covered quite a bit of ground towards the
planet’s current biota. The oceans, lakes, and rivers are filled
with fish, many of which are indistinguishable from fish of today,
although the proportions of the different classes are different;
there are more lobe-finned fishes (the ancestors of land animals)
than today. Many of the land and water arthropods are also fairly
modern-looking, one of the obvious exceptions being the trilobite, an
ancient arthropod reaching back to the Cambrian but whose days are
severely numbered by this time. Another difference is that some of
them grow to amazing sizes; there are, for example, dragonflies with
wing spans close to a meter across. The land is already populated by
a wide variety of amphibians and reptiles, although especially for
the latter, they don’t resemble those of today very much at all;
some of the latter are the so-called mammal-like reptiles, named for
exactly the reason you imagine, and indeed all modern mammals are
descended from a lineage of them. Many ferns, mosses, and conifer
trees also fill the landscape along with other plants, though none of
them sport flowers or are pollinated by insects or birds – for that
matter, there are no birds or other flying vertebrates to pollinate
anything. As for the state of the world, much of its climate at this
time is hot and arid, and all of the major land masses are joined
together in a single, super-continent named Pangaea, the center of
which is possibly the largest desert ever to have existed in Earth’s
history.
It
was on this world that what has been called the Mother of Mass
Extinctions then happened 250 million / three months, the
Permian-Triassic, or P-T, extinction event. Over the next several
million years (about one or two days on our timeline) the great
majority of species in both land and marine environments vanish from
the fossil record, leaving no descendants. Estimates of the total
carnage are as high as 90-95% of marine species, and 70% of land. Of
most important for this discussion were the extinction of most of the
mammal-like reptiles (excepting the one that gave rise to mammals, of
course), leaving room for the rise of the group of reptiles called
archosaurs, which appeared either somewhat before or after the
extinction, depending on their exact definition. Whatever definition
is accepted, the archosaurs are the ancestors not only of modern
crocodiles and birds, but also the dinosaurs and pterosaurs (the
flying reptiles of the dinosaur age), and the marine reptiles which
would come later.
The
first true dinosaurs appear in the fossil record some 225-230 million
years ago, in the middle of the Triassic, the first period of the
Mesozoic Age, or age of reptiles. And thus finally we are here, a
mere two and a half months ago on our timeline, with our planet being
95% of its current age. So the dinosaurs aren’t ancient at all,
but are more like our next door neighbors in time. The next major
extinction event, at the end of the Triassic 205 million years ago,
cleared out most of the rest of the dinosaurs’ competition and led
to their being the main land animals on Earth for the next 140
million years, until they themselves met their end in the most famous
(but not largest) extinction event of all, the Cretaceous-Tertiary or
K-T event, 65 million years / 24 days / 1½% of Earth’s age ago.
Indeed, looked at this way it seems like just yesterday that
Tyrannosaurus Rex and its cousins spread their terror across the
land, making the world of Jurassic Park almost sound
plausible.
* * *
The
causes of mass extinction events are still strongly disputed in the
scientific community. One of reasons for this is that there are many
possible contenders for causes, geologic, climate-related, or
astronomic; so many, in fact, that we probably should count ourselves
absurdly fortunate that life was never completely wiped out by one or
a combination of them by now – assuming that that there isn’t
something other than fortune going on here. Over the last 20-30
years, however, two particular contenders have come to acquire
particular interest: asteroid impacts and massive volcanism known as
flood basalt events. Interestingly, although both scenarios are very
different in the events that begin them, the consequences to Earth’s
biota are due largely to changes they cause in the planet’s
atmosphere. The impact scenario, which was first hypothesized for
the K-T (dinosaur) extinction by Alvarez et al in 1980,
proposes that a large (5-10 miles across) asteroid struck Earth 65
million years ago, injecting so much dust, water vapor, soot, and ash
into the atmosphere (the latter two from massive forest fires caused
by burning debris from the impact) that the amount of sunlight
reaching ground level would have been seriously reduced for as much
as several years. The results would have been severe cooling and
reduction in photosynthesis, killing all those plants and animals
that could not adapt to these extreme changes.
The
Alvarez hypothesis is supported not only by geologic data such as
high iridium concentrations and shocked quartz in the K-T boundary
stratum, but also by the discovery of an impact crater of appropriate
the right size in 1979 just off the Yucatán peninsula. Despite all
this, the hypothesis is not without its problems, one of the main
ones being an alternative which might be better (another problem is
that there are known large impacts in the geologic record that are
not associated with mass extinctions). This alternative is the
massive volcanism / flood basalt event hypothesis.
Volcanic
eruptions occur in different scales and varieties. Some, like the
volcanoes of Hawaii, involve a more-or-less steady flow of lava from
an underground magma reservoir over a long period of time. Others
are explosive, releasing large amounts of hot ash and gasses, along
with pyroclastic flows and lava and rock “bombs” over a
relatively short time period. Mount Vesuvius, which destroyed the
Roman cities of Pompeii and Herculaneum in a 79 AD eruption, is one
example of this latter type of volcano. Another is Mount Tambora in
Indonesia, whose eruption in 1815 was probably the largest in
history: not only did it outright kill over 71,000 people, it
injected so much gas and dust into the atmosphere that there was
significant global cooling over the next several years, killing many
more from famine due to crop failures.
I
mention Tambora not because it was so horrific a catastrophe, but
because compared to many eruptions in our planet’s geological
history, even its recent history, Tambora was a firecracker which
possibly didn’t result in a single species’ extinction. When it
comes to serious volcanic eruptions once again we should breathe a
sigh of relief and count our lucky stars. And worry about what could
happen in the future.
If
you want an example of what not only could be but one day will be,
though nobody knows when, take a trip to Yellowstone National Park.
While you are admiring the many geysers, hot springs, fumaroles, and
steam-belching mud pots, and standing in awe of the massive yellowish
volcanic tuffs whose color gives the park its name, imagine what lies
under your feet that is causing all these natural splendors. What
you are standing on is an enormous volcanic caldera, one
approximately 1500 square miles in extent (the Tambora caldera is
about 20 square miles), which itself overlays a magma chamber of
comparable size. The caldera is the result of a colossal volcanic
eruption, or “supervolcano”, some 640 thousand years ago, which
was orders of magnitude larger than the 1815 Tambora event. If such
an eruption were to happen today, which it very well could, the human
death toll would be easily in the millions if not tens of millions,
from both direct and indirect (climatic, etc.) causes. And there
have been eruptions of comparable magnitude that occurred more
recently. The Lake Toba caldera on the Indonesian island of Sumatra,
comparable in size to Yellowstone, exploded around 70,000 years ago
in an eruption so large that it may have almost caused all human
species existing at that time to go extinct.
Yet
Yellowstone and Toba are nowhere near the upper limit when it comes
to volcanic events on this planet. Flood basalt events involve
single or multiple volcanic eruptions (of any type) so enormous that
they cover up to a million square miles and more of Earth’s surface
with lava, to a depth of a mile or more. Such eruptions can dwarf
supervolcanoes as much if not more than the latter dwarf any historic
volcanoes. Actually, a good example of flood basalts is the maria,
or “seas”, on the moon. These are the dark, relatively
crater-free, regions which cover much of our satellite world (I
should say the half of that world that always faces us due to tidal
locking; the “far” side of the moon is almost devoid of maria).
The two largest of these events on Earth, within recent geologic
history, are the Deccan flood basalts of India and the Siberian flood
basalts. I don’t want to even speculate what would happened if
either of these events occurred today, because what is so interesting
about them is their timing: the Deccan basalts were laid down
approximately 65 million years ago, and the Siberian basalts around
250 million years. Both dates coincide with what are probably the
two most important mass extinction events: the Deccan with the K-T
event which finished the dinosaurs, and the Siberian with the P-T
event, the largest extinction event in the geologic record. If you
find it difficult to believe that such timing is just coincidence,
you have a great deal of company, including me.
* * *
For
myself, the on-going debates about the causes of mass extinctions is
wonderful and exciting because it shows science, and the people who
devote their lives to it, at its best. The combination of simple
human inquisitiveness, wide-eyed imagination, the struggle to avoid
dogmatism, and hard-nosed skepticism based on details has no equal in
any other form of human endeavor. We see an enormous mystery, we
open our minds as wide as lotus blooms to come up with possible
solutions, but the solutions we come up with must run the gauntlet of
data (often incomplete, unclear, and contradictory), experimentation,
calculations, and competition from alternatives that other minds as
bright as our own have dreamed up while we were busy proving our own
ideas. And then, just when we think we’re on the verge of having
it all figured out, someone or something else – usually new data –
comes along to re-ignite the controversy.
Until
1980 the whole field of explaining mass extinctions was, from what I
can tell, rather moribund. It wasn’t that there was a dearth of
ideas on what could wipe out such a large proportion of species over
a short period of (geologic) time. The two main problems, again from
my own reading, is first, a lack of detail in the fossil record, in
terms of exactly which species went extinct and exactly when they did
so – for example, had an extinction occurred abruptly or over
several millions years? – and second, the difficulties in
determining what kind of evidence a certain kind of event would leave
– an example in this case would be, if a nearby supernova
(exploding giant star) had caused the extinction event, what markers
in the geologic record would reveal it? The result was that the
extinction events looked more like the random acts of a god or gods
instead of anything that could be explained scientifically. Many
scientists didn’t even want to think about them, even.
The
main effect of the Alvarez impact hypothesis, more than anything else
if I am reading history right, has been to bring the spotlight, not
only of publicity but of serious scientific thinking on the whole
subject of explaining mass extinctions. What makes this achievement
all the more amazing was, it wasn’t what the Alvarez’s were
trying to do at all! Many people by now have heard the story of how
they stumbled upon anomalously high levels of iridium in samples of
K-T boundary clays, inferred a large asteroid impact from that single
fact, then suggested that said impact might be the reason for the
dinosaurs’ mysterious vanishing act 65 million years ago. What
most people probably don’t know is how much resistance the impact
hypothesis met among paleontologists and biologists at the time, many
of whom argued that the fossil record showed a gradual decline of
dinosaur numbers and diversity for millions of years leading up to
their final extermination – data that the impact scenario decidedly
does not predict – and that in any case it was hard to
reconcile the pattern of extinctions with the consequences of a large
impact. What is more interesting, to me at any rate, is how these
objections didn’t go away even as further evidence – shocked
quartz, soot from mass fires in the boundary clays, and the discovery
(or re-discovery) of the 65 million year old “Chicxilub” crater
in the Yucatan peninsula – appeared the cinch the case for an
impact-caused extinction.
Scientific
analogies are a bit treacherous, but in some ways this one reminds me
of Neils Bohr’s work on explaining the hydrogen atom in 1913 with
the then new ideas coming out of quantum theory: it was brilliant
and original, predicted properties of atomic hydrogen perfectly such
as its spectral line series, was critical in the development of
atomic theory, and rightfully earned Bohr his fame and Nobel Prize,
but …but that was all it did. It couldn’t predict the properties
of any other atom or molecule, nor did it offer any explanation for
why electron orbits should be quantized. It was an absolutely
necessary and essential step, but that’s all it was, a step.
By
the 1990’s impact theory was so in vogue that some scientists were
attempting to explain all extinction events with them. It was
even postulated that the sun had a stellar companion, suitably dubbed
Nemesis, that was too dim to have been detected so far and which had
a highly elliptical orbit that brought it close to the inner solar
system once every 26 million years or so, disturbing enough comets in
the Oort cloud and / or Kuiper Belt to rain destruction down on Earth
and other planets. The 26 million year cycle in extinction events
didn’t hold up well under statistical analysis, however, but that
didn’t stop people from finding iridium anomalies and shocked
quartz and other geologic evidence associated with the big die-offs.
Still,
a lot of the paleontological evidence just could not be made to fit
simple impact hypotheses, leading some scientists to look for other
causes. And here I think they were helped by yet another one of
those convergences like those that Schmidt mentions in his book. The
1990s and 2000s were times of increasing scientific work on the
effect of anthropogenic greenhouse gasses on Earth’s climate, and
the possible implications of quick temperature rises of around 5° –
10° C or more, rises that seem plausible given our current state of
knowledge. One of these implications involves the release of a large
amount of methane, another potent greenhouse gas, from methane
“clathrates” or “hydrates” lying on the ocean floors (these
are methane deposits held in an essentially frozen state in water ice
by the cold and great pressure of the ocean depths; once brought to
the surface they disintegrate rapidly, releasing the methane into the
atmosphere). A large scale release of this methane, which exists in
enormous quantities on the bottoms of northern and southern oceans,
could lead to even more drastic, short-term warming of the planet.
This
issue led to a reexamination of the possible consequences of
large-scale volcanic eruptions on our planet’s climate, in
particular the Deccan and Siberian flood basalt eruptions which are
by far the largest over the last 500 million years ago – er, six
months on our timeline, if anyone is still counting. Eruptions do
release large quantities of various gasses, mostly water vapor (H2O),
carbon dioxide (CO2), methane (CH4), sulfur
dioxide (SO2) and hydrogen sulfide (H2S); all
of these are greenhouse gasses, though some, like SO2, can
lead to cooling also due to the sulfate aerosols they often form; in
addition, H2S can also erode the ozone layer in large
enough quantities. We’ve already seen how a large eruption in the
historic record, Tambora in 1815, had serious consequences for
Earth’s climate for several years. What might enormously large
eruptions on the scale of the Deccan and Siberian lead to?
It
isn’t an easy question to answer, in part because it is difficult
to determine the quantities of volcanic gasses released in these
eruptions and the time period(s) they were released over, let alone
the global effects of such releases. But by the mid 2000s some
scenarios had been worked out which showed how these events could
have played a major role in the P-T and K-T extinctions which
coincided with them. Also, by then evidence for these scenarios had
been found in the geologic and fossil records.
What
follows is worrisome to me because of what it might imply about our
current problem of anthropogenic greenhouse gas warming. In my
chapter on the future, I placed man-made environmental catastrophes
aside in considering our own future evolution while stressing that
something like Homo sapiens and our technological civilization
on this planet is a gigantic experiment which may never have been
played out in the universe before (and may never again), so we don’t
know how it is really going to turn out. There are simply too many
unknowns, and trying to wade through all possibilities is a Sisyphean
task a hundred books wouldn’t be enough to cover. But if the
connection between mass extinctions and massive flood basalt
eruptions proves out to be true, what it suggests about our current
greenhouse warming is sobering. We may need to take much more
drastic steps than we are taking now in order to prevent any
significant warming.
What
makes the flood basalt driven mass extinction hypothesis especially
interesting is that it brings together a sequence of different
environmental triggers to accomplish its devastating effects. First,
large scale emission of greenhouse gasses, such as H2O,
CO2, CH4, SO2, H2S (the
last gives rotten eggs their odor), and others warm not only the
atmosphere over a period of thousands of years or more, but also the
upper layers of the oceans and other bodies of waters. Note that as
mentioned, this warming may have released large amounts of methane
(CH4) from methane hydrates on the ocean floors, further
exacerbating this warming. As warming water reduces its ability to
dissolve gasses, reduced O2 in the oceans leads to
widespread anoxia, which in combination with increased acidity due to
CO2 absorption, the direct effect of killing off many
organisms in these layers followed. The indirect effect of reduced
O2 / increased CO2 is far more insidious,
however. In different areas of the ocean and some seas (the Black
Sea of today is a well-known example), dissolved O2 only
reaches a certain depth; the waters below this depth, known as the
chemicline, support only anaerobic organisms, some of which are
bacteria which produce copious amounts of H2S. This H2S
normally never makes it to the surface, but the anoxic waters and
die-offs of aerobic organisms during the flood basalt events may have
resulted in the H2S producing bacteria proliferating and
the chemicline rising to the surface, releasing large amounts of this
gas into the atmosphere. The effects of high concentrations of H2S
in the atmosphere would be catastrophic for most land dwelling
organisms. Not only is this gas directly toxic, more so even than
the hydrogen cyanide (HCN) used to kill concentration camp prisoners
in Nazi Germany, it is also light enough to rise into the
stratosphere, where it would poison the ozone layer, allowing in
higher levels of lethal ultraviolet light from the sun.
If
the flood basalt / greenhouse warming / ocean anoxia / H2S
producing bacteria increase scenario is correct, it should leave
certain evidence in the geological / fossil record. And indeed, for
the P-T event we do find multiple forms of evidence, in the form of
biomarkers for these bacteria in oceanic sediments, in lower oxygen
levels in the atmosphere, in the patterns and types of extinctions,
and in fossils showing the effects of increased ultraviolet
radiation. So it would appear that the Siberian flood basalt events
hypothesis of P-T extinctions is well on its way to being confirmed.
But what about the effects of the Deccan floods basalts on the K-T
extinctions 65 million years ago? The scenario is essentially the
same, but can they too account for that event, with or without a
large impact?
The
Deccan flood basalt caused K-T extinction was actually first proposed
by Dewey McLean more-or-less concurrently with the Alvarez impact
hypothesis. Although initially overshadowed by the more dramatic
image of an asteroid striking Earth and the possible consequences of
such an event, this may be due more to yet another convergence in
scientific thinking; for this was about the same time that the
“Nuclear Winter” hypothesis was being popularized by Carl Sagan
and other scientists, and the parallels between the two ideas were
undeniably striking. Sagan’s Nuclear Winter hypothesis claimed
that even a relatively small nuclear war (relatively truly being a
relative term in this case) could release into the atmosphere so much
dust and ash and soot from massive firestorms that photosynthesis
could be blocked for up to several years and the planet thrown into a
deep-freeze, very much like the results of a large impact.
Naturally, the re-discovery of the Chicxulib crater also lent a great
deal of support to the Alvarez impact theory.
In
the end, scientific hypotheses rise or totter and fall based on
physical data, in this case the data being the precise timing of the
K-T extinctions compared to that of the Chicxilub impact and the
Deccan flood basalts. And what appears to be happening over the last
ten years or so as far as I can see is that the timing is coming to
favor the flood basalt hypothesis better and better. The Chicxilub
impact may have actually occurred several hundred thousand years
before the final extinction K-T extinction pulse. Of course, it is
possible that the impact did still play an important rule, in driving
some species closer to extinction. But it is becoming difficult, at
least from my read of the controversy, to doubt the central
importance of volcanism.
* * *
It’s
high time to return to our timeline, although all this talk about
mass extinctions and their possible causes leaves me hungry for more.
We left off at the end of the dinosaurs, the K-T event, some 65
million years / 24 days / 1½% of Earth’s age ago if I may
reemphasize just how recent the dinosaurs are in Earth’s history.
We have entered the Cenozoic Age, or age of mammals. The first
mammals actually appear in the Triassic, along with the dinosaurs,
but until this point they have remained relatively small, mostly
nocturnal animals, who eked out their existence in ecological niches
the dinosaurs, for whatever reasons, never penetrated. The
extermination of their dominating cousins in the K-T mass extinction
finally allows the mammals to multiply, diversify, and grow into
those now empty niches (along with the dinosaurs’ closer cousins,
the birds). Remember that on our timeline Earth and the solar system
condensed 4½ years ago and the universe began almost fourteen. We
have come a long ways indeed.
How
far back can we trace our own ancestors, genus Homo and the
australopithecines or upright-walking apes they evolved from? No
evolutionary line can be said to start anywhere, of course, but we
can identify certain points as being of special importance. For
humans, this is probably the split between the human and chimpanzee /
bonobo evolutionary lineages which, according to a combination of
genetic and fossil evidence, began somewhere between six and seven
million years ago, somewhere in Africa. This is one tenth the
distance back to the K-T extinction, so we are speaking of about two
and a half days ago on our timeline or 0.15% of Earth’s total age.
What
about modern humans? This depends on your exact definition of
modern. Humans that look essentially modern, but from an
intellectual point of view probably weren’t there yet appear
several hours ago, while those ancestors we would call fully modern
probably about two hours before the present. These modern humans
first leave Africa to colonize the rest of the world not much more
than a half-hour ago, establishing the first proto-civilized
settlements within the last four or five minutes. The rise of the
modern industrial state takes to some thirty seconds before the
present, and you, depending on your age, probably between ½ and 2½
seconds old, have another couple of seconds or so to go before it’s
lights out – well, that may very well depend on how some of the
predictions I made in the last chapter pan out. Very comforting, no
doubt.
And
now let us head into the more distant past, the earliest moments of
the future, or what most people refer to as the Big Bang.