Oxygen
Page 9
No creature is as synonymous with lowliness as the worm, but its humbleness belies quite a complex design. To burrow through mud requires muscles, and in order to contract, these must be opposed by some form of ‘skeleton’ — in a worm’s case a body cavity filled with fluid.
Muscular contraction demands oxygen, and as this cannot diffuse through more than a millimetre or so of tissue, the early worm-like animals must also have had a circulatory system and a mechanism for pumping oxygenated fluid, such as a primitive heart. To move forward at all, the body segments of the worm must have contracted in a coordinated sequence, and this in turn implies at the least a simple nervous system. To displace sediment while worming forwards demands a mouth, a gut and
Snowball Earth, Environmental Change and the First Animals • 57
an anus; and indeed some fossilized trails do contain pellets that are interpreted as of faecal origin. Some worms may have been predators of a sort, and to hunt might have been equipped with eyes, or light-sensitive spots, as are their descendants. In short, at a rudimentary level, these primitive worms must already have evolved many of the features that are needed by large animals that can move around. The worms were also bilaterally symmetrical (the same on either side) and segmented — two central features of the later Cambrian animals. It seems likely, then, that our earliest animal ancestors were an approximation to a worm, as Darwin’s critics saw only too well when they satirized his view of Man’s descent.
For all its lowliness, a worm is much too complicated a creature to have arisen overnight; other, earlier, fossils have indeed been found, dating back to about 600 million years ago — nearly 60 million years before the Cambrian explosion, a period of time as long as that from the extinction of the dinosaurs to the present. Most of the fossils of these earliest multicellular animals are equivocal to say the least: faint, circular impressions, sometimes a centimetre [about ½ an inch] across, but unrecognizable as animals in any conventional sense. Beyond this, nothing. If there were indeed any animals large enough to be visible to the naked eye before about 600 million years ago, they must have had an uncanny knack of avoiding fossilization. The existence of a longer Precambrian fuse can only be deduced from the evidence of molecular ‘clocks’, perhaps the most powerful and controversial of tools available to the molecular palaeontologist. Molecular clocks imply that the evolution of multicellular animals — the metazoans — may have stretched back to at least 700 million years ago, and possibly to more than a billion years ago.
Molecular clocks make use of the genetic differences between present-day species to predict the time since their divergence from a common ancestor. When an ancestral species splits off new species, these new species and their descendants all gradually accumulate different genetic changes — mutations in the DNA — over time, that eventually make them very different from each other. As different, for example, as humans are now from fruit flies. The basic assumption is that species drift away from one other, in terms of their shared genetic inheritance, at a steady rate. At face value, this assumption is of course nonsense — we have crossed a lot more evolutionary space over the last 600 million years than have worms, for example. The difficulty is to specify a distance across evolutionary space on the basis of averaging the rate of evolution in dif-
58 • FUSE TO THE CAMBRIAN EXPLOSION
ferent species. Luckily, a few simple tricks can be applied, allowing a more robust guess to be made. Two of the most essential factors are calibration of the molecular clock using reliably dated fossils and the use of an average rate of genetic drift obtained by determining the changes in a large number of different genes in a wide variety of species. The evolutionary biologist Richard Fortey provides a nice analogy in his delightful book Trilobite! He compares molecular clocks with an old-fashioned horologist’s shop, in which hundreds of clocks beat to their own music of time. Some have stopped completely, others tell wildly different times, but the majority indicate that the time is around two-thirty in the afternoon. While the onlooker may doubt the exact time, he will probably be satisfied that it is mid-afternoon. Similarly, the results of molecular clock calculations sometimes vary by hundreds of millions of years, according to the genes and species studied, but all indicate a substantial fuse of animal evolution during the Precambrian. The overall weight of evidence implies that the fuse lasted at least 100 million years, and possibly as much as 500 or 600 million. If this is true, the earliest animals must have been too small to leave visible fossils, so the search is on for tiny impressions measuring less than a millimetre [1/16 in] across.
The genetic studies reveal more than a Precambrian fuse, however.
They also indicate that an ancient set of genes, which controls the embryological development of all animals today, were already fully operational in the earliest Cambrian animals. These genes are known as the Hox genes. They are remarkable in two ways. First, there are relatively few of them: just a handful of genes control many of the steps in the early development of all animal embryos — from flies to mice and men. Second, the Hox genes of different species have very similar coding sequences. Even distinct groups, such as the arthropods and the chordates — the group to which we and other vertebrates belong — share sets of very similar Hox genes. Let us consider the implications of these two points in turn.
How is it that so few genes can control embryological development?
The Hox genes function as master switches along the length of the body, switching on or off the hundreds of other genes required to make, say, a leg or an eye, depending on the position in the body. They behave like opinionated newspaper proprietors, who influence the tone or coverage of their papers on particular issues, such as politics or European union. If the proprietor buys another newspaper, with a different political affiliation, he might bring about a shift in their political reporting to reflect his own views. A single rogue proprietor is enough to make the paper transfer
Snowball Earth, Environmental Change and the First Animals • 59
its affiliation from right-wing to left-wing overnight. In the same way, if a master-switch Hox gene responsible for growing an eye in a fruit fly is switched on, by mistake or design, in a body segment further back, it alters the way other genes are switched on or off in that segment, causing bizarre developmental errors, such as the growth of an eye on a leg.
Normal development thus requires a set of master-switch Hox genes plus a regulatory framework that ties the action of a given Hox gene at a particular position in the body to a particular effect on the genes under its influence.
Why are the Hox genes so similar in different species? The fact that animal groups that were already distinct in the Cambrian (such as the arthropods and chordates) share very similar Hox genes implies that all inherited them from a common Precambrian ancestor. This is purely logical reasoning. It is highly unlikely that all the Cambrian species evolved exactly the same genes independently. We might as well suggest that the physical characteristics we share with our brothers and sisters — light hair, blue eyes, white skin, or brown eyes, dark hair and black skin — have nothing at all to do with inheritance and everything to do with a common environment. It is conceivable that Cambrian species transferred genes to each other by lateral transfer, by sex, but for this to happen, utterly different species would have to exchange genes in a way that could not be imagined today. If a lobster were to copulate with a jellyfish, it is hard to imagine a successful outcome. It is more reasonable to assume that the Hox genes were in fact inherited from a common ancestor shared by all Cambrian animal species. If that was indeed the case, then the Hox genes — the basic genetic tool-kit needed to produce segmented body parts, such as a head with feelers and eyes on either side — must have evolved before the Cambrian explosion. Along with the fossil findings, this genetic evidence constrains the significance of the Cambrian explosion. It was not the evolutionary diversification of the first multicellular animals, which probably happened more than 600 million years ago; nor wa
s it a radiation of relatively large animals, which took place among the stuffed bags of protoplasm, the Vendobionts, 570 million years ago.
No, the Cambrian explosion was above all a diversification of segmented bilateral animals similar to modern-day crustaceans.
According to the Harvard University palaeobiologist Andrew Knoll and his University of Wisconsin colleague, the molecular biologist Sean Carroll, the Cambrian explosion was probably driven by a rewiring of the regulatory loops between the master Hox genes and the genes under their
60 • FUSE TO THE CAMBRIAN EXPLOSION
control. Shuffling and duplication of Hox genes allowed existing genes to take on new responsibilities. The number of Hox genes correlates roughly with morphological complexity. Thus, nematodes have one cluster of four Hox genes (and are simple in structure), whereas mammals have 38
Hox genes arranged in four clusters. Goldfish, rather surprisingly, have 48
Hox genes in seven clusters; biology never stoops to a perfect correlation.
In essence, though, duplication of Hox genes allows the replication and subsequent evolutionary modification of repetitive body parts. Having extra, dispensable, body parts makes specialization and complexity easier to achieve. For example, in the ancestors of the arthropods, the large group to which modern insects and crustaceans belong, a small change in the workings of a Hox gene could cause new legs to sprout on previously bare segments, and these then evolved into antennae, jaws, feeding appendages and even sexual organs.2 While many of the fine genetic details are being worked out step by laborious step, the outstanding question has shifted from ‘how’ to ‘why now?’, or rather, ‘why then?’ The broad answer offered by Knoll and Carroll is that the Cambrian explosion was the historical product of an interplay between genetic possibility and environmental opportunity. The most likely environmental gate-keeper was oxygen.
The long equanimity of the earth, which had persisted since the upheavals of around 2.3 to 2 billion years ago, was shattered for a second time by another series of snowball Earths, starting about 750 million years ago. This time the cataclysm was not a singular event, caused by the exhaustion of a greenhouse gas such as methane, but a 160-million-year roller-coaster ride, comprising possibly as many as four great ice ages, two of which, the Sturtian (at around 750 million years ago) and the Varanger (at around 600 million years ago) were arguably the most severe in Earth’s history.
We don’t know exactly what triggered this dramatic sequence. The most plausible explanation argues that the tectonic meanderings of the continents happened to bring about their freak assembly around the 2 One reason why the segmented bilateral body plan is so pregnant with genetic potential is that small changes in Hox genes, shifting their zone of responsibility, can lead to sudden and dramatic changes in morphology – a purely Darwinian stepwise process that is easily mistaken for a giant leap over genetic space.
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Equator for a time.3 This would have meant that all the land masses on Earth were free of ice. To understand why this should matter, we must look at what happens when rock is exposed to the air, or to warm oceans with plentiful carbon dioxide. Rock can be eroded by dissolved carbon dioxide, which is weakly acidic. As a result of this reaction, carbon dioxide is lost from the air and becomes petrified in carbonates. But when glaciers form over land, the underlying rock becomes insulated from the air by the thick layer of ice. This means that the rate of rock erosion by carbon dioxide is cut to a fraction and the carbon dioxide stays in the air. In fact, in such a situation, carbon dioxide actually builds up in the air, because it is also emitted more or less continuously from active volcanoes.
Over aeons of time, such a build-up can make a very considerable difference, unless it is offset by erosion of rocks. Because carbon dioxide is a greenhouse gas, this build-up produces an increased greenhouse effect.
The surface of the Earth gets warmer. Global warming ultimately halts the spread of the glaciers from the poles. So in today’s world, where there are large land masses at or around the poles, any spread of the polar glaciers towards the Equator is offset by a greenhouse effect that gets stronger whenever the glaciers advance, and weaker whenever they retreat.
Now consider what happens if polar ice forms over the oceans instead of the continents. This is what may have taken place to form the late Precambrian snowball Earth. Because the continents were clustered together in the tropics, glaciers at the poles formed over sea only. These polar glaciers could not affect the rate of rock weathering on the continents. The rocks kept on drawing down carbon dioxide from the air.
Atmospheric carbon dioxide levels began to fall. The gradual draw-down of carbon dioxide had an anti-greenhouse effect, encouraging the spread of the glaciers. There was nothing to stop the advance: the equatorial continents kept sucking up more and more carbon dioxide. Worse still, as the glaciers marched on towards the Equator, they reflected back the Sun’s light and heat, cooling the planet still further, sending the Earth into a vicious spiral of cooling. Eventually, the whole Earth was covered in ice. The ice reflected back so much of the Sun’s heat that the planet was in danger of turning into an eternal snowball. Yet Earth is not a snowball today. Somehow, the spell was broken; what happened?
When the equatorial continents were finally sealed beneath the ice, the continuous draw-down of carbon dioxide by rock weathering ceased.
3 Palaeomagnetic studies do in fact support this arrangement, albeit with a wide margin of error.
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With no liquid water exposed to the atmosphere, there was no evaporation, no rain. Any carbon dioxide in the air stayed in the air. All climatic traffic between the air and the frozen seas and the buried rocks came to an end. Deep beneath the surface of the Earth, however, the forces of vulcanism were oblivious to the icy crust. Active volcanoes burst through the ice, spewing volcanic gases into the air, among them carbon dioxide.
Over millions of years, carbon dioxide accumulated in the air again, re-warming the Earth. Finally the glaciers began to melt. As this happened, more of the Sun’s warmth was retained, less reflected back. The vicious circle of reflectance went into reverse. But there was a diabolical catch. The juxtaposition of the continents around the Equator continued to set the same snare: the whole crazy snowballing and melting repeated itself as many as four times before the continents were finally dispersed to the four corners of the Earth by the forces of plate tectonics.
This story is admittedly hypothetical, but Joseph Kirschvink (whom we met in Chapter 3) and others have dispelled any doubt that glaciers did encroach to within a few degrees of the Equator at that time. Their grand synthesis is supported by the presence of so-called cap carbonates in the rocks of Namibia and elsewhere. Cap carbonates are exactly what their name implies: belts of limestone, at times hundreds of metres thick, which cap the glacial deposits laid down during and immediately after the ice age. For many years, their intimate relationship with the glacial deposits seemed a paradox, as carbonate rocks normally form only in warm oceans and in the presence of plentiful carbon dioxide; and neither of these circumstances are compatible with ice-age conditions. A solution to the conundrum was put forward by the Harvard University geologists Paul Hoffman and Dan Schrag in 1998. They argued that a build-up of perhaps 350 times the current levels of carbon dioxide would be required to melt the ice. Once the reflectance of the snowball Earth had been overcome in this way, however, the extreme levels of carbon dioxide would swing the global climate from an ice-box to an oven in a matter of a few hundred years. Searing temperatures, tropical storms and torrential rain would scrub carbon dioxide from the skies, turning the oceans into an acid bath. The only way to regain a normal chemical balance would be to drop carbonates out of the oceans, straight on top of the glacial debris, so forming the cap carbonates. Thus Hoffman and Schrag use the cap carbonates themselves as proof of the plausibility of a snowball Earth.
Geologists continue to squabble over the plausibility of a snowball Earth.
How did life survive? Could bacterial production of methane — another greenhouse gas — have helped to melt the ice before carbon dioxide levels became so high? Were the oceans completely sealed from the air under the deep ice? Or was the Snowball Earth perhaps more of a Slushball Earth, in which the oceans never completely froze, and icebergs floated on open water in equatorial regions.
Even though we don’t yet know how severe the snowball glaciations really were, geochemists can add up the consequences, as recorded in the isotope signatures of the rocks. These signatures tell a fascinating story of their own. In particular, the ratio of carbon-12 to carbon-13 (see Chapter 3, page 34) in the cap carbonates and other rock formations veers from background volcanic levels to the highest levels of carbon-13 in the entire Precambrian (Figure 3). For carbon isotopes to plateau at background volcanic levels, there can have been almost no burial of organic matter, as burial of organic matter always disturbs the natural equilibrium left behind in the oceans. If there was no organic matter buried, then there must have been next to no organic matter produced — in other words, no biological activity. This stark conclusion is the geological equivalent of a flat-line cardiac trace, and is interpreted as the near-extinction of all living things either during or immediately after each ice age, when carbon dioxide was being scrubbed from the skies and the oceans were an acid bath. Conversely, a swing to the highest levels of carbon-13 in the whole Precambrian implies a massive production and burial of organic carbon, (mostly derived from microplankton, algae and bacteria), which left behind an excess of carbon-13 in the oceans, to form the next layer of carbonate rocks. At such peak times — after the Sturtian glaciation some 700 million years ago, for example (see Figure 3) — life was flourishing as never before.