Mailund on the Internet

You probably didn’t miss all the buzz about Ardipithecus yesterday.  If you did, here’s a few links to get you started on all the press releases and blog comments:

• The 4.4 million year old Ardipithecus ramidus (Anthropology.net)
• Ardipithecus ramidus remains finally published after years delay.  Not all that interesting (Greg Laden)
• Ardipithecus FAQ (John Hawks)
• Humans vs chimps — neither is an offshot (Genomicron)
• Ardipithecus round-up (Evolving Complexity)

…and that is just a start.

For the original papers, you get a list here:

• Science publishes 11 papers on Ardipithecus ramidus (Anthropology.net)

I haven’t started reading those yet — I still find paleonotology hard to read — but I am going to spell my way through them in the weekend.

From what I can read from all the second-hand comments, though, we are talking about a 4.4 mya species with lots of traits similar to modern humans as opposed to modern chimpanzee hinting that our common ancestor might not necessarily be more chimp-like than human-like.

This may or may not surprise you…  That depends a bit on how special you think humans are from the African apes (chimp and gorilla).  A priori, traits shared by those two and not humans are probably evolved on the human lineage rather than independently evolved on both the chimp and gorilla lineages.

A quick glance at the two might give you the impression that chimps and gorillas are very much alike, but with more careful observation a lot of traits are actually quite different, and in many cases they do not share traits while we have another to any higher degree than one of those have a unique trait.

The time back to our shared ancestor with chimps is just as long for the chimp lineage as for the human lineage, so it makes sense that they have evolved just as much as we have, when you think about it.  The time back to the gorilla is essentially the same, since the split with the gorilla and the split with the chimp are very close in time, so exactly the same argument goes there.

Traits that Ardipithecus share with humans include being omnivore — but then chimps eat fruit and gorillas eat leaves so you cannot really say that they share a trait that we do not here anyway — and more interestingly Ardipithecus might have been bipedal.  To some extend, anyway, but not nearly as much as Lucy.

Now bipedalism is a trait that is usually strongly associated with humans, but even here the story is a bit more complex.  The African apes are knuckle-walkers (the orangutan is not), but this trait might have evolved twice, so I don’t really have a problem with bipedalism being the ancestral trait and knuckle-walking being derived…

Anyway, whether this fossil is “shocking” or just interesting, it is certainly an important piece in the puzzle of our evolution.

The one thing that bothers me right now is the speciation time question.  If Ardipithecus is 4.4 million years old and on the human lineage, how far back do we find the human-chimp speciation?

Our own estimate, based on genomic comparison, would actually put this speciation event more recent than that, at 4.1 mya, but this estimate is slightly biased and correcting for that we get up to about 4.4 mya.

Still too recent if Ardipithecus is well along the line leading to humans…

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I read this paper on our way back from Leipzig and then again today to see if I missed anything in the first read through (I was pretty tired at the time).

Detecting ancient admixture and estimating demographic parameters in multiple human populations

Wall, Lohmueller and Plagnol, Mol Biol Evo 26(8):1823-1827

We analyze patterns of genetic variation in extant human polymorphism data from the National Institute of Environmental Health Sciences single nucleotide polymorphism project to estimate human demographic parameters. We update our previous work by considering a larger data set (more genes and more populations) and by explicitly estimating the amount of putative admixture between modern humans and archaic human groups (e.g., Neandertals, Homo erectus, and Homo floresiensis). We find evidence for this ancient admixture in European, East Asian, and West African samples, suggesting that admixture between diverged hominin groups may be a general feature of recent human evolution.

What they do in this paper is to fit a two population coalescent model, with expansion, migration, bottlenecks and the works, to both an African+European and an African+Asian data set, then use this fitted model as a null model of the genetics of the populations.  They then 1) do a test on an LD statistic against this null model, taking rejections of this null model as evidence for admixture from archaic humans, and 2) fit an admixture extension of the model to estimate the level of admixture.  They find evidence for admixture with archaic humans for both data sets, with a somewhat higher degree in the Europeans.

I’m a bit underwhelmed by the paper, I must admit.  I’m not saying that there is no admixture with archaic humans, but this approach does not convince me.

Even when taking various demographic effects into account in the modeling, the null model is unlikely to exactly fit real data.  Taking deviations from the null model as any kind of evidence for admixture thus seems a bit hasty.

Not that I have any better ideas as to how to approach this, just, in my eyes the jury is still out on the question of admixture with archaic humans…

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Wall, J., Lohmueller, K., & Plagnol, V. (2009). Detecting Ancient Admixture and Estimating Demographic Parameters in Multiple Human Populations Molecular Biology and Evolution, 26 (8), 1823-1827 DOI: 10.1093/molbev/msp096
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Found this in search engine hit in my stats this morning:

Nope “chimpanzees more chromosomes”.

Compared to humans anyway.

Our chromosome 2 is chimp chromosomes 2a and 2b (thus named after our chromosome).

Actually, it is us that have one less chromosome, really.  The gorillas and orangutans also have chromosomes 2a and 2b, and our chromosome 2 has bits of telomere inside it, so all evidence points to that our chromosome 2 is a fusion of two chromosomes.

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Lecture at YouTube:

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Just read this post on my good friend Thomas Widmann’s blog: A body that requires a brain:

One thing about human evolution that has been puzzling me for years is that there are several areas where the human body seems to be dependent on a highly developed brain

He then mentions head hair and nails – that needs to be cut – and human buttocks – that needs to be wept for fæces.

In short, I can’t see how human head hair, nails and buttocks could have evolved before the human brain was sophisticated enough to invent combs, knives and toilet paper.

My guess is that those features are results of sexual selection, but the question remains if we evolved these features before or after we had the means to deal with them.

Our modern brain seems to be more recent than our body – in the sense that we see anatomically modern humans earlier than we see art or complex tools – but do we have any knowledge about when features like these evolved?

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