What environmental conditions led to the evolution of the forerunners of animal life? New discoveries of the 1980s and 1990s have given us a much clearer view of the early Earth during the great evolutionary transitions we saw in the last chapter. The Earth's earliest life seems to have formed during or soon after cessation of the heavy comet bombardment. By about 3.8 billion years ago that heavy cosmic bombardment ended, and by 3.5 billion years ago we find the first fossilized evidence of life.
The region that has yielded Earth's oldest fossils found to date is known to Australians as the North Pole, because even in this isolated continent, it is uniquely remote and inhospitable. The rocks in this region belong to a unit of interbedded sedimentary and volcanic rocks known as the Warrawoona Series. Geologists have deduced that the deposits were consolidated in a shallow sea over 3.5 billion years ago. There is the evidence of storm layers and evidence as well that on occasion, a hot sun evaporated small pools of seawater into brine deposits. But it is not these structures that have created so much excitement about the Warrawoona rocks. This ancient bit of Australia holds the world's oldest stromatolites, low mounds of lime and laminated sediment that have been interpreted as the remains of microbial mats—in other words, life.
Stromatolites (the "stone mattresses" we mentioned earlier as an anomaly, multicellular prokaryotes) are the most conspicuous fossils and the most commonly preserved evidence of life for more than 3 billion years of Earth history; they provide our best record of early life. They have been found on every continent in rocks half a billion years old and older. Today, they are found in only one type of environment on Earth, in quiet, briny tropical waters. Such environments are refuges from algal grazers; stromatolites can no longer exist on most of our planet's surface because they would quickly be eaten. The photosynthesizing bacteria termed cyanobacteria are modern equivalents of these ancient deposits.
The presence of stromatolites is a sure clue that by 3.5 billion years ago, life on this planet had left its earliest, probably hydrothermal or deep-earth environments and diversified onto the surface of the planet. For a billion years the prokaryotes were masters of the world, but life was still scattered. According to the fossil record, it was not until about 2.5 billion years ago that the organisms that produced stromatolites had released sufficient quantities of oxygen to form sedimentary deposits known as banded-iron formations. Prior to the appearance of common stromatolites, there was no dissolved oxygen in the sea, no gaseous oxygen in the atmosphere, and hence no possibility of mineral oxidation. With the appearance of oxygen, however, large volumes of iron that had been dissolved in seawater precipitated out as it oxidized into iron oxides—rust, in other words. Today, there still exist at least 600 trillion tons of such iron oxides deposited before 2.5 billion years ago in these banded-iron formations.
The time interval commencing about 2.5 billion years ago is marked by a change in the very tectonic nature of the planet Earth—its rate of mountain building and continental drift. By this time, the heat production from radioactive elements locked in Earth's rocks had diminished, for some of the radioactive elements decayed rapidly early in Earth's history. This material was like a finite amount of fuel within the interior of the planet, and as it was used up, heat flow declined. It turns out that the processes of continental drift and mountain building are by-products of heat rising from within Earth, and as the amount of heat decreased over time, so did these two activities. There is also some evidence that around this time, a major pulse of land formation occurred, allowing larger continental land masses to form. As the new continents formed, many shallow-water habitats were created, and these proved favorable environments for the growth of photosynthesizing bacteria. We can speculate that from about 4 billion to about 2.5 billion years ago, there were few large continents, but numerous volcanic island chains dotted the world. After 2.5 billion years ago, continental land masses began to form, and volcanism on a global scale lessened.
With this increase of habitat, ever more stromatolites grew and flourished. This in turn relentlessly pumped ever more oxygen into the sea. As long as there was dissolved iron in the seawater, all of the liberated oxygen was quickly locked up in the banded-iron formations. By about 1.8 billion years ago, however, this reservoir of dissolved iron material was used up. We know this because after that time, no more banded-iron formations were laid down. This changeover left an indelible mark on the sedimentary record of Earth, for as the sea became saturated with oxygen, the time of banded-iron formations ended forever—or at least until some far-distant future when our planet may again no longer have oxygen. With nowhere else to go, oxygen began to emerge into our planet's atmosphere and, in so doing, probably gave life its first impetus toward animal life.
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