When was the chimp-human last common ancestor?

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What’s the difference between a monkey and an ape? Monkeys have tails on the outside, whilst apes have tailbones on the inside. Our coccyx places us firmly in the ape family; closely related to chimps. We were the same species at one point but eventually, we split. There was a chimp-human last common ancestor; after which we parted ways (mostly).

This marked the dawn of the human family. The origins of the hominins.

As such, pinpointing when the chimp-human last common ancestor lived is quite a big deal. Unfortunately, few fossils exist from this period. In fact, the closest fossil to this crucial moment, Sahelanthropus, is so incomplete and ambiguous there’s a debate on which side of the divide it’s from¹!

So people have been turning to our genes instead, hoping that identifying a genetic “clock” can shed light on when our family emerged. For more than 20 years, this research indicated that split happened ~7 million years ago. This seemed consistent with the fossil evidence, explaining why the Sahelanthropus fossils from then were so ambiguous. Everyone was happy².

However, there’s a growing body of evidence this is dramatically under-estimating the age of the chimp-human last common ancestor; with some suggesting that we may have split from them up to 14 million years ago³.

How to find the chimp-human last common ancestor

Pinpointing this last common ancestor seems like a fairly simple process. All you need to know is how many mutations develop each generation, and how many mutations humans have developed since that split. Then it’s a simple process of calculating the number of generations that have passed and, from that, years³.

However, this simple-sounding method masks a lot of complexity.

• Generation time has changed over the course of our evolution, as we now mature slower than chimps³.
• Some parts of the genome may be more ‘susceptible’ to change, with 20% of mutations occurring close to another⁴.
• The father’s age adds 2 extra mutations per year in humans, but 3 in chimps. The extra age mutations in chimps mean that the father is responsible for 90% of the mutations in kids but only 75% in humans⁴.

This is why genetic dates often come with a margin of error of upwards of millions of years. There’s a lot of complicated factors making it hard to work out! It’s also why you should be super-skeptical of most attention-grabbing genetic dates, as many will pick from the extreme ends of this margin of error to make their findings seem sexy.

A new (old) date for the chimp-human last common ancestor

However, all of those confusing factors aren’t impossible to study. They just take time, since apes have this annoying habit of living for ages.

Nevertheless, thanks to decades of research we have a better understanding of primate generation lengths than ever before. So, back in 2012 Langergraber and colleagues examined all this new data to come up with the most accurate date for the chimp-human last common ancestor yet³.

Specifically, their new figures for generation length indicated that mutations accrued half as fast as we thought. This pushes back the divergence of humans and chimps to ~14 million years ago³!

Crucially, their figures suggested this split was a gradual process and could’ve lasted until ~8 million years ago³. For all that time chimp and human ancestors may have been genetically close enough to continue to mix. Kind of like what we were doing with Neanderthals more recently.

Speaking of Neanderthals

Langergraber et al. also applied to their data to other key splits in the human lineage. For instance, they found the Neanderthal-human last common ancestor was also older than we thought. We likely diverged between 600,000-400,000 years ago (prior estimates placed it closer to 400,000-200,000 years ago)³.

Interestingly, this appeared to resolve some long-standing controversy in the fossil record. There are actually some possible Neanderthal ancestors ~400,000 years ago, which would be way too early under the old dates. Pushing the divergence back makes this controversy disappear³.

However, not everyone thought these new results prompted a re-writing of the textbooks. Partly because that’s a lot of faff, but also because there was some uncertainty over these new results. Notably, Langergraber et al. were using human mutation rates, combined with generation data from other apes. Is crossing the streams really valid⁵?

These are real concerns that the initial authors acknowledged⁶. Whilst more data has been gathered, which it turns out is consistent with an older date for the chimp-human last common ancestor⁴, the lack of ape generation rates remained a concern.

Apes together strong

So, in 2019 Besenbacher et al. set out to fix this problem by getting actual figures for ape mutation rates. That way, we can be sure these new dates aren’t barking up the wrong (family) tree⁷.

In a dramatic plot twist, they found that the other apes wound up with far more mutations per generation than we did. When these figures are fed into the calculations, they suggest that the chimp-human last common ancestor lived closer to the original date, 6-7 million years ago⁷.

As well as re-re-dating our split from chimps, this also implies that the human mutation rate has been slowing down. This is a weird, fascinating conclusion that may have something to do with changes in when we hit puberty⁷.

But that’s not what we’re here for today. We’re here to see our family tree, and figure out when we split from chimps. Well, here’s the latest version based on all the best data⁷.

Of course, given how hard genetics can be (and as a result, how much these figures have changed) I wouldn’t be too surprised to see this get updated another time. Or 10.

 References

1. Wolpoff, M.H., Senut, B., Pickford, M. and Hawks, J., 2002. Palaeoanthropology (communication arising): Sahelanthropus or’Sahelpithecus’?. Nature, 419(6907), p.581.
2. White, T.D., Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C.O., Suwa, G. and WoldeGabriel, G., 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science, 326(5949), pp.64-86.
3. 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
4. Oliver Venn, Isaac Turner, Iain Mathieson, Natasja de Groot, Ronald Bontrop, Gil McVean. 2014. Strong male bias drives germline mutation in chimpanzees. Science, 344(6189):1272-1275
5. Gibb, G. C., & Hills, S. F. (2013). Intergenerational mutation rate does not equal long-term evolutionary substitution rate. Proceedings of the National Academy of Sciences, 110(8), E611-E611.
6. Prüfer, K., Langergraber, K. E., Pääbo, S., & Vigilant, L. (2013). Reply to Gibb and Hills: Divergence times, generation lengths and mutation rates in great apes and humans. Proceedings of the National Academy of Sciences, 110(8), E612-E612.
7. Besenbacher, S., Hvilsom, C., Marques-Bonet, T., Mailund, T. and Schierup, M.H., 2019. Direct estimation of mutations in great apes reconciles phylogenetic dating. Nature, pp.35-6.