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Old Mike
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« on: 15 July 2009, 15:59:22 pm » |
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From New Scientist July 15 2009.
WHEN Darwin unveiled his theory of evolution, the earliest known fossils lay in rocks belonging to what Darwin called the Silurian age. Older rocks seemed devoid of fossils. The apparently sudden appearance of sophisticated animals such as trilobites did not fit in with Darwin's idea of gradual evolution.
"If my theory be true, it is indisputable that before the lowest Silurian stratum was deposited... the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer," Darwin wrote in the first edition of On the Origin of Species. His conundrum is known as Darwin's dilemma.
Of course, we have since discovered innumerable fossils from far earlier periods. Rocks as old as 3.8 billion years contain signs of life, and the first recognisable bacteria appear in rocks 3.5 billion years old. Multicellular plants in the form of red and green algae appear around a billion years ago, followed by the first multicellular animals about 575 million years ago, during the Ediacaran (see "The rise of animals").
Perplexing questions
Even so, many perplexing questions remain. Why did animals evolve so late in the day? And why did the ancestors of modern animals apparently evolve in a geological blink of an eye during the early Cambrian between about 542 and 520 million years ago? A series of recent discoveries could help explain these long-standing mysteries. These findings suggest that the first animals evolved far earlier than we thought, perhaps more than 850 million years ago. The really extraordinary part, though, is that these early animals may have completely transformed the planet, paving the way for the larger and more complex animals that followed them.
Some of the biggest finds have come from an ancient seabed in China, called the Doushantuo Formation, where unusual conditions preserved some extraordinary fossils. Layers between 550 and 580 million years old, during the last part of the Ediacaran, contain tiny spheres consisting of anything from one to dozens of different cells - just like the early embryos of animals. Some have suggested they are the remains of giant bacteria, but a series of studies over the past decade have left little doubt that they really are animal embryos.
In 2007, for instance, Leiming Yin of the Nanjing Institute of Geology and Paleontology in China reported finding embryos encased inside hard, spiky shells - unlike anything produced by bacteria. What is more, shells that are identical apart from the lack of preserved embryos on the inside can be found in rocks as old as 632 million years - the dawn of the Ediacaran - suggesting that the animal embryos themselves go back this far.
Other, more tentative findings push the dawn of animals back even further. Roger Summons of the Massachusetts Institute of Technology and his colleague Gordon Love analysed brownish, oily sandstone cores drilled from as deep as 4 kilometres below the desert of Oman. The oil is what's left of dead organisms that drifted down to the depths of ancient oceans, where they decayed slowly due to the lack of oxygen. No visible fossils remain, but within that oil are molecular fossils - chemicals derived from the ancient organisms. In layers between 635 and 713 million years old, Summons and Love found 24-isopropylcholestane (24-IPC), a stable form of a kind of cholesterol nowadays found only in the cell membranes of certain sponges. "The biomass of sponges must have been pretty substantial," says Love, now at the University of California, Riverside. "They were ecologically prominent."
First animals
Another recent discovery places the first animals even earlier. In 850-million-year-old rocks in the MacKenzie Mountains of Canada, researchers were exploring the remains of a massive reef of stromatolites built by cyanobacteria. Among the stromatolites, they found peculiar netted patterns of grey calcium carbonate interspersed with petrified mud. Team member Elizabeth Turner, a palaeontologist at Laurentian University in Sudbury, Ontario, recognised the patterns as a characteristic of a collagen mesh - something only animals build (Geology, vol 37, p 475).
As some dead sponges decay, the scaffold of collagen protein that holds their cells together is replaced by specific calcium carbonate minerals. Meanwhile, mud fills the spaces in between. These characteristic patterns occur during the last 500 million years, and have even been produced from dead sponges in the lab.
What Turner found looks very primitive. "It's even simpler than a sponge," she says. "This may represent the basal metazoan," she says, "the simplest form of animal life, in which a few different types of cells were living together in a shared, collagenous matrix."
These findings fit in well with the molecular evidence. By comparing the genomes of different living organisms, biologists can get an idea not only of what their common ancestor was like but also when it lived. Such studies suggest that the first animals were sponges, or something very like them.
Molecular clocks
What's more, molecular clock estimates have always placed the origin of animals well before the Cambrian. "Even the most conservative molecular clock estimates suggest that animals began to differentiate six, seven, even 800 million years ago," says Andy Knoll, a palaeobiologist at Harvard University. "This goes a long way toward reconciling the geologic record with molecular clock estimates."
The case for early animals is not yet rock solid. It is possible the 24-IPC was produced by single-celled organisms, points out Martin Brasier of the University of Oxford, and he is not convinced by Turner's interpretation of the carbonate patterns. Nevertheless, he thinks animals probably did evolve early on.
But if the first animals really did appear so early, why did they leave behind so few traces? A clue comes from the spiky-shelled Doushantuo embryos. They closely resemble the dormant embryos, or diapause cysts, produced by modern starfish, sea anemones, jellyfish, sponges and other animals when conditions are unfavourable, such as when oxygen levels fall too low.
This is significant because recent studies of the minerals in ancient seabeds around the world suggest that the ocean remained a hostile place for far longer than was once thought. From around 2.5 billion years ago, the poisonous waste produced by photosynthetic bacteria - oxygen - was piling up in the atmosphere and seeped into the top layer of the oceans. Contrary to what has generally been thought, though, everything deeper than a few metres remained anoxic. And having oxygen in the atmosphere but not in the deep oceans created another problem. Oxidative weathering of sulphurous minerals on land, coupled with rain and run-off, poured sulphur into the oceans. Bacteria below the oxygenated surface layer then converted the sulphur into hydrogen sulphide, a gas highly toxic to complex cells called eukaryotes, from which all plants and animals evolved.
Double whammy
The double threat of anoxia and hydrogen sulphide would have confined eukaryotes to the upper few metres of the oceans. Any storm or upwelling that brought deeper water to the surface would have killed everything in its path. "As long as that sulphidic floor existed," Knoll says, "those events would be a persistent brake on eukaryotic evolution."
Sulphide also created another problem for eukaryotes. It latches onto dissolved molybdenum, copper and zinc, forming insoluble minerals that would have locked these essential trace elements away in the sea bed. Shortages of these elements would have limited the fixation of nitrogen, starving entire ecosystems - and especially eukaryotes - of nutrients.
Many bacteria, on the other hand, thrived in these conditions. In fact, by consuming whatever oxygen did penetrate the depths and by pumping out sulphides, they helped maintain this hellish regime. "Bacteria would have run the oceans," Knoll says. "Eukaryotes would have been uninvited guests."
Then, sometime between 900 million and 500 million years ago, everything changed. A series of super ice ages - so-called Snowball Earth events - gripped the Earth, possibly giving early animals a foot in the door. What's striking about Love's fossil sponge chemical, 24-IPC, is that it blinks on just after the Sturtian ice age. Love sees it as no coincidence.
Ocean chemistry
"This glaciation reset the chemistry of the oceans," he says. Ice caps covering the continents halted delivery of sulphur to the oceans and cut off production of hydrogen sulphide. "You have changes in ocean chemistry like an increased availability of molybdenum and zinc," says Ariel Anbar, a biogeochemist at Arizona State University in Tempe, "all of which play into making the world more hospitable for eukaryotes and ultimately, metazoans."
Sponges or something like them would have been the first animals on the scene. They lack a nervous system and have no need for circulatory systems. Animals like jellyfish might also have evolved early.
Life was still very precarious for these early animals, though. Producing diapause cysts may have been essential for their long-term survival, perhaps enabling some to survive even after an upwelling of anoxic water had wiped out every active animal, says Knoll.
The lack of oxygen in the oceans might also explain why we have found so little evidence of early animals. The bigger and thicker an animal's body, the more problems it would have had getting enough oxygen. "It may well be that any mutation that made you large killed you," says Knoll.
Armourless
The same applies to the evolution of armour. For primitive animals without gills or a circulatory system, any hard coat would limit the diffusion of oxygen into the body.
So the first animals were almost certainly tiny and soft-bodied, and thus left few fossil traces. Nevertheless, they changed the oceans forever - by eating its rulers, the bacteria. By doing so, they would have introduced selective pressure for organisms to get larger, to avoid being eaten. And when large plankton die, they often settle to the bottom and are buried, rather than decaying near the surface and consuming oxygen in the process. Slowly, one ocean basin after another was transformed as oxygen penetrated their depths. "When animals started hoovering the ocean out, everything changed, including the ventilation, oxygenation and clarification of the oceans," says Nick Butterfield, a palaeobiologist at the University of Cambridge.
He draws an analogy with modern bodies of water, such as Chesapeake Bay in the US, that become dominated by anaerobic bacteria. Once they're established, these anoxic systems resist change. Often, only the introduction of filter feeders such as mussels can restore clear, oxygenated water. "In the absence of that top-down grazing, you can't get out of that," he says.
Fuel of life
As as the oceans changed, the stage was finally set for the evolution of more sophisticated body forms. The idea that rising oxygen levels played a key role in the explosion of life in the Cambrian is far from new, but most researchers attribute the rise in oceanic oxygen to an increase in the atmosphere. If Butterfield is right, it was actually due to animals taking over from bacteria. "These geochemical signatures [of oxygenation] are not what's causing the evolution of animals," he argues. "They're consequences of the appearance of animals."
"I think he is right," says Brasier. In fact, he thinks the link between complex life and the transformation of the planet runs even deeper. In Darwin's Lost World, a book published earlier this year, Brasier suggests that the increased burial of carbon resulting from the rise of large cells and groups of cells - perhaps including plants such as seaweed too - sucked carbon dioxide out of the atmosphere, triggering the series of ice ages that helped the first animals wrest control of the oceans from bacteria. "Rather than being the cause of animal evolution, the ice ages may well have been the response to it," he says.
Rather than being the cause of animal evolution, the ice ages may well have been a response to it
For a while the climate bounced between wild extremes: during warm periods complex life thrived and lots of carbon was locked away, leading to deep ice ages. During the ice ages, carbon burial ceased, and the planet warmed again. These swings ended only when burrowing creatures with a gut evolved towards the end of the Ediacaran, Brasier thinks. By recycling the organic matter falling to the sea floor, they reduced carbon burial and stabilised the climate. "There are no Snowball Earth glaciations after big animals evolve," he says.
But if the evolution of animals really did trigger the ice ages and the oxygenation of the oceans, rather than the other way round, why didn't animals appear much sooner? After all, single-celled eukaryotes were around from 1.5 billion years ago, and possibly much earlier.
Sheer difficulty
Butterfield thinks the main reason animals evolved relatively late in Earth's history was the sheer difficulty of evolving the cell adhesion and signalling machinery necessary for cells to work together. Once these basics were in place, though, the pace of evolution began to quicken.
Put together, all the recent findings and ideas paint a picture of early evolution dramatically different from what we long imagined. The oceans did not suddenly become hospitable places for large animals 2.5 billion years ago when the atmosphere began to fill with oxygen, nor did animals suddenly appear during the early Cambrian. Instead, the first animals appeared much earlier but were limited to a thin layer of surface water in hostile oceans still dominated by bacteria. They were restricted in size by the lack of oxygen, starved of vital nutrients and regularly killed en masse by toxic upwellings.
Their deaths were not in vain, though. As their bodies sunk to the bottom of the toxic seas and were buried, carbon dioxide was sucked out of the atmosphere, triggering a series of deep ice ages that reset the chemistry of the oceans. The surviving animals seized the opportunity to wrest control of the oceans from the bacteria, producing clear waters rich in oxygen in which larger, more complex animals could evolve. Thus the stage was set for the Cambrian explosion.
Of course, fossil hunters are going to have to do a lot more digging to confirm these startling new hypotheses. "I suspect things will turn up," says Brasier, "but if they don't we have to listen to the evidence."
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