Revolutions that Made the Earth by Lenton Tim; Watson Andrew; & Andrew Watson

Revolutions that Made the Earth by Lenton Tim; Watson Andrew; & Andrew Watson

Author:Lenton, Tim; Watson, Andrew; & Andrew Watson
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2013-07-14T16:00:00+00:00


PART IV

THE COMPLEXITY REVOLUTION

11

Life gets an upgrade

Now we have reached halfway through the history of the planet. It is 2.3 billion years ago, early in the Proterozoic Eon. There is oxygen in the atmosphere, albeit somewhat less than today. The puzzle is; why does it take another one and a half billion years or so to produce large, complex life in the form of animals? The sudden appearance of complex forms in the lowest Cambrian was a puzzle that Darwin saw as one of the principal objections to his theory of evolution. He correctly guessed that the Precambrian must actually have covered a vast amount of time and that ‘during these vast periods the world swarmed with living creatures’, though he could offer no explanation as to why they left no fossils (1). We now know that the majority of older fossils are microscopic and unicellular, and for this reason had not been recognized. However, despite now having a much richer (though admittedly still patchy) fossil record of those distant times, the earliest complex forms have only been pushed back a little, into the latest part of the Proterozoic, around 600 million years ago. Clearly a long and cryptic road had to be travelled to evolve complex life, and there is still a yawning chasm to bridge.

The intervening long spell of stability, especially from 2 to 1 billion years ago, has been christened the ‘boring billion’. Evolution appears to have proceeded at a sedate pace, whilst the oceans and atmosphere were locked in a halfway house state, which may have impeded life. Or at least that is how it appears, given a rather sparse record of those distant times. It may be that the full biological and chemical richness of that distant world remains hidden. From what we already know, ‘boring’ does not really do it justice. The ‘stable’ or ‘slow’ billion would be a better description of the Proterozoic Earth system. What we can say with some confidence is that when things eventually changed, they did so radically. Complex life appeared in a time of great turbulence for the planet as a whole—known as the Neoproterozoic—that included changing oxygen levels and extreme ice ages.

In this part of the book we examine this complexity revolution of the Earth and what led up to it. We start in this chapter by considering the biological aspects. In particular, how did eukaryotes evolve fromprokaryotes? And what are the key features of eukaryotes that would later allow them to produce large, complex, differentiated, life forms? In the next chapter we assess; when did eukaryotes evolve? Then in Chapter 13 we turn to the atmosphere and especially the ocean, to see what evidence there is that the Earth system got locked in an unproductive state. We look at the mechanisms that could have held oxygen levels down, and how the unusual ocean of the time may have inhibited life. Finally in Chapter 14 we take a whole system view of the Neoproterozoic, and explore some



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