Changes in DNA result in variations among offspring (chapter 11). As a result, variant organisms achieve greater or lesser reproductive success depending on interactions with their local population, with partners in symbiosis, and with their environments (chapter 12). These processes have produced the vast diversity of organisms from a common ancestor. It has taken an immense amount of time for them to do so.
Fossil and radioisotope dating evidence along with comparative DNA sequencing tell us that life on Earth has had a continuous history of at least 3,800 million years, some 80-85% of the age of the planet. For the first two thirds of that time, all organisms were prokaryotes. For 70% of the time there were no multicellular organisms. Roughly 85% of that vast period had elapsed before any recognisable animals appeared, 90% had passed before there were land plants, and 99.9% before the earliest hominids. Future discoveries might alter these estimates but they are approximately correct. However, numbers cannot convey a mental picture of the huge time-scales involved. The following diagrams might be more helpful; the scales are thousands of millions of years before (minus sign) or after (plus sign) the present.
Our sun is a second-generation star. That is, the sun and its planets, including the Earth, were made from debris from the disintegration of firstgeneration stars, which were formed after the Big Bang. The chemical elements necessary for life as we know it, such as carbon and oxygen, were made as the first-generation stars neared the ends of their lives. The sun was formed about 5,000 million years ago and the planets during the succeeding 500 million years. A further 500 million years probably elapsed before the Earth could support life, which it has continued to do ever since. It seems likely that the Earth will go on supporting life for another 1,500-2,000
million years. A star such as the sun survives for about 10,000 million years altogether, so about half its life-span has elapsed so far.
These facts are relevant to the origin of life on Earth and elsewhere, matters to which we shall turn in chapters 14 and 15. But for the present chapter we need a scaled-up version of the highlighted segment of Fig. 13-1: the interval between the origin of life on Earth and the present day is shown in Fig. 13-2.
What have been the most salient events in this 3,800 million year history? What we consider "salient" is largely a matter of viewpoint, and our viewpoint is unavoidably anthropocentric. It also depends on the evidence available to us. The right-hand part of Fig. 13-2 (nearest the present day) is relatively crowded. This might be because more has happened recently, as organisms have grown more diverse and complex. But there is a simpler reason: we know a lot more about recent times than ancient ones. The diagram is unbalanced not (necessarily) because very little happened during the first 70% or so of life's history, but because we are largely ignorant of it.
To illustrate this, consider our classification of rocks on the Earth's surface into geological eras: Cambrian, Ordovician, Devonian and so on. These eras segment the last 600-700 million years, the segments becoming smaller as they approach the present. Everything older than that - in other words, 85% of the history of the planet - is labelled "Precambrian" We can only base our classifications on evidence, including evidence of what was living when the rocks were formed, and we have very little basis for subdividing the "Precambrian".
The way we describe periods of mass extinction also shows how selective our knowledge is. There is compelling fossil evidence that five major periods of extinction have punctuated the history of life. These were intervals of little more than a million years during which large percentages of existing species were wiped out. In the third and biggest of the five, which occurred at the end of the Permian era, 96% of all known species disappeared. In the wake of each of these catastrophes, the ecological vacuum was filled by newly-evolved species and the Earth was re-populated with novel organisms. If we locate these five major extinctions on our second time diagram, we obtain the following picture:-
Does this seem credible? Were five major extinctions packed into 430 million years, while no extinctions occurred during the preceding 3,400 million? It is surely more plausible that regular mass extinctions have occurred, roughly every hundred million years. But we only have evidence for the last five, not the previous thirty or so.
The conclusion is inescapable. Our attempts to reconstruct the first three-quarters of the history of life on Earth are based on very scanty knowledge, and are probably wildly inaccurate.
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