Our lineage’s fossil record is especially sparse between 4 – 8 million years ago. From that time period we have roughly 50 fragmented specimens making up only 6 or so individuals. It isn’t much to go on, which is particularly disappointing since genetic data indicates that humans and chimps diverged during this period. Indeed, with no archaeology from this time period (the archaeological record doesn’t start until ~2.6 million years ago) genetic data is pretty much the only source of information on the aforementioned split.
Yet genetics doesn’t provide that much information either, or at least what it does tell is fairly uncertain. By looking at how different two species genomes are (in this case humans and chimps) you can use the mutation rate to estimate how long has passed since they diverged. Such research indicates that humans and chimps split between 5 – 7 million years ago. The aforementioned uncertainty stems from the fact that studying genomes in the deep past is riddled with “confounding variables” (uncertainties in the methodology which cannot be controlled for).
For example, if you know how many mutations accrue in each generation you can work out how many generations have passed since we split. However, you still have to work out the length of generations in order to work out how many years this has been. This is complicated by the fact that chimps mature faster than us, meaning our ancestors also likely had different generation lengths. But what were they? We can try and work it out based on how fast fossils appear to have matured during life, but there is still a fair bit of uncertainty here. This makes our end result less accurate.
As such it should be no real surprise that these dates are subject to change as new ways of compensating for these uncertainties arise (along with new techniques, technology etc.). Science, as ever, is an adaptive process striving to be as accurate as possible and nowhere is this more obvious than our changing view of genetic divergence. The earliest calculations placed this split ~5 million years ago but refinements since then have produced the aforementioned figure.
Despite the fact that change is to be expected, the results of a recent revision of divergence dates has proved very surprising indeed. Scientists extensively studied chimps and gorillas to come up with more accurate information about how long their generations are. They also analysed the genomes of whole families of apes to work out the rate at which mutations occurred time. This gave them the most accurate information on generation span and the rate at which mutations occur to date. They also performed similar studies on humans, allowing them to recalculate when the various species all diverged.
Their results cut the previous estimate of mutation rate in half, pushing back the divergence of humans and chimps to ~14 million years ago. This split was a gradual process and could’ve lasted until ~8 million years ago! Their recalculation also applies to other splits, suggesting humans and neanderthals diverged between 600,000 and 400,000 years ago (compared to the prior calculation of between 400,000 and 200,000 years ago). Both of these recalculations fit with the fossil data, resolving extensive controversy.
Many neanderthal-esque specimens have been dated to ~400,000 years ago and there has been great debate over whether they are truly neanderthal or a very close neanderthal answer (since neanderthals shouldn’t really exist at that point under the old dates). Pushing the divergence back makes this controversy disappear. Similarly, the earliest members of the human lineage (dating to 4 – 6 million years ago) already look decidedly un-chimp like. Although a lot of this probably stems from the fact our last common ancestor with the chimps also looked quite un-chimp like they already appear to have evolved some unique traits of their own. This mystery could be easily explained by the fact they’d already been evolving these traits for an extra few million years.
However, whilst it fixes some controversies it also generates others. Even older fossils, from when orang-utans started to diverge already fit quite well into the existing timeline. Changing that timeline makes these fossils appear anomalous, creating a freshly baked batch of controversy. These issues could be resolved by the fact it appears the mutation rate has been slowing down in the ape lineage, meaning in the very deep past the existing timeline would still be accurate. One potential cause of this could be an increase in brain size. Larger brains take longer to grow, thus increasing generation length and so the rate at which mutations occur). Of course, whether such a slowing down is even happening is still being debated.
Luckily that does not detract from the recalculation of the recent timeline. Multiple studies are reaching the same conclusion and the fossil evidence – as I’ve mentioned – is consistent with it. It would certainly seem like humans and chimps started to diverge ~14 million years ago, and all the other divergence dates should be doubled too. This is the biggest revolution in human origins research for many years.
|Langergraber KE, Prüfer K, Rowney C, Boesch C, Crockford C, Fawcett K, Inoue E, Inoue-Muruyama M, Mitani JC, Muller MN, Robbins MM, Schubert G, Stoinski TS, Viola B, Watts D, Wittig RM, Wrangham RW, Zuberbühler K, Pääbo S, & Vigilant L (2012). Generation times in wild chimpanzees and gorillas suggest earlier divergence times in great ape and human evolution. Proceedings of the National Academy of Sciences of the United States of America, 109 (39), 15716-21 PMID: 22891323|
A lack of internet has been the cause of my non-posting, but now it’s back and so am I!