Miocene hominids and a crisis of confidence29 Jun 2007
Out of this week's Science Times special on evolution, I clicked into John Noble Wilford's article first, titled "The Human Family Tree Has Become a Bush With Many Branches".
Now, I don't know about you, but that seems like a boring headline to me. They've been talking about human evolution being a bush for going on 20 years now. It was an old idea when I was in graduate school. So it seems like, if this is all we have going on, the "new frontier" of paleoanthropology must be pretty dull.
The writer doesn't write the headlines, and the headline doesn't describe Wilford's story, which is basically a verbal slide show of fossil discoveries over the last decade or so. Some bone pictures (of the actual species discussed) accompany the article, and it's a good enough sort of account of new finds since 1990, framed around the tension between fossil finders and molecule mavens.
But I'll be a little critical. The thesis is that paleoanthropologists suffered a crisis of confidence after molecular data came online in the 1980's, and "a rapid succession of fossil discoveries since the early 1990's has restored" it.
Well, OK, maybe. But consider the listed discoveries: Kenyanthropus, Ardipithecus ramidus, Ardipithecus ramidus, Orrorin tugenensis, Sahelanthropus tchadensis, Homo floresiensis, and Australopithecus anamensis. Of all of these, only Ar. ramidus and Au. anamensis have gone without significant controversy.
We can set aside H. floresiensis for a moment -- the controversy about it being possibly the loudest, it also stands apart as the only species listed younger than 3.9 million years. All of these early Pliocene and Miocene species have also been challenged -- by the discoverers of the others, by old hands, and by young upstarts like me. At least one research group has claimed that all of the Miocene "genera" may actually belong to one species. Another has claimed that most of these "hominids" may actually be apes.
Whether there was any crisis of confidence among paleoanthropologists, all this disagreement is certainly business as usual.
And, contrary to the article, every one of these species would be thrown from the hominid line, if we believe the molecules. Here's the text from the article:
Genetic clues also set the approximate time of the divergence of the human lineage from a common ancestor with apes: between six million and eight million years ago.
Fossil researchers were skeptical at first, a reaction colored perhaps by their dismay at finding scientific poachers on their turf. These paleoanthropologists contended that the biologists' "molecular clocks" were unreliable, and in some cases they were, though apparently not to a significant degree.
The new finds have filled in some of the yawning gaps in the fossil record. They have doubled the record's time span from 3.5 million back almost to 7 million years ago and more than doubled the number of earliest known hominid species. The teeth and bone fragments suggest the form -- the morphology -- of these ancestors that lived presumably just this side of the human-ape split.
It is true that the new fossils date as far back as 7 million years; with Sahelanthropus just under that date, Orrorin at around 6 million, Ar. kadabba at 5.5, Ar. ramidus at 4.4, and Au. anamensis at around 4.1.
But it has been many years since a genetic comparison indicated a human-chimpanzee common ancestor as old as 6-8 million years. This year's study by Holbolth et al. (2007) estimated a human-chimpanzee speciation time of 4.1 +/- 0.4 million years. That makes Au. anamensis possibly too young to be a hominid. The rest of those species would presumably be just so many apes.
Now, I don't believe for a second that Au. anamensis is an ape and not a hominid. It just looks too much like Au. afarensis -- so much so that some would put them in the same species. The evolutionary transition between these two is well documented, and will be more so when some as-yet-unpublished fossils come out. So anything younger than 4.1 million years is almost certainly not right for the human-chimpanzee divergence.
But the 4.1 million year estimate is not unusual compared to other recent studies. My post from last May covers many of these recent studies, including last year's problematic "hominid-chimpanzee hybrid speciation" paper by Nick Patterson and colleagues. The conclusion in that paper about hybridization was certainly wrong, but the date of 5 million years was right in line with other estimates.
These genetic comparisons are not easily dismissed. Possibly there has been a rate deceleration of mutations in the human lineage that means that the estimated dates are too recent. Maybe 4.1 million years can be stretched into 6 million. Maybe it can even be stretched into 7 million. But all this stretching does have other effects -- on the estimated dates of earlier divergences -- and those are compounded by a large multiple of the few million years we may try to push the human-chimpanzee speciation date. That 4.1 million year estimate is calibrated from an African-Asian great ape divergence at 18 million years ago. Push the human-chimpanzee divergence to 7 million, and you push the orangutan-human divergence back into the Oligocene. Are silent sites in humans evolving more slowly than cercopithecines? Probably. Are they evolving that much slower than orangutans? I suppose nothing is impossible, but maybe we should take another look at those fossils.
All this is to point out that there really is a conflict between these Miocene "hominids" and genomic evidence about human-chimpanzee speciation time. I don't see any magic solution to this problem from the molecular side -- those dates keep coming up again and again from different regions, and from comparisons across many regions -- including estimates that are not calibrated by other fossil divergences. This is not an easy "the molecular clock must be wrong" kind of problem.
Nor are the fossils an easy problem. There is pretty good evidence for vertical posture or hindlimb-dominant movement in all of these "hominids." Up to now, we've accepted these kinds of features as de facto evidence of bipedality, and assumed that bipedality is such a unique character of hominids that it is unlikely to be any older. Yet few of these fossils provide really good evidence for obligate bipedality, and some of them provide none at all.
Is it possible that bipedal apes long preceded the divergence of humans and chimpanzees? Was the common ancestor of the two lineages a biped? Or was significant vertical posture a common feature of many Miocene apes -- making Sahelanthropus a possible homologue of Oreopithecus?
Which feature is the important one? The long nuchal plane of Sahelanthropus? The femur neck cortical bone distribution of Orrorin? The toe bone of Ar. kadabba? Heck, I can hardly convince my undergraduates about that toe bone!
I've talked to people about this. Some think that all the molecular stuff is just jibberjabbing, and any day now we will find out that the date estimates were wrong all along.
I think it may be time to start doubting our confidence again.
UPDATE (6/28/2007): I've gotten into rather an interesting e-mail discussion about whether I should have included Homo georgicus on the list of new species. Frankly it didn't occur to me: Wilford didn't mention it.
Actually if you start to think about all the new names that have been proposed in the last 15 years, it is a quite bushy list. It will be no surprise that I think this bushiness has more to do with the listers than the listees.
Anyway, there is something interesting about early Homo right now that goes beyond the simple splitter/lumper questions. I'll have more to say about it in a few days.
Hobolth A, Christensen OF, Mailund T, Schierup MH. 2007. Genomic relationships and speciation times of human, chimpanzee, and gorilla inferred from a coalescent hidden Markov model. PLoS Genet 3:e7. doi:10.1371/journal.pgen.0030007
Patterson N, Richter DJ, Gnerre S, Lander ES, Reich D. 2006. Genetic evidence for complex speciation of humans and chimpanzees. Nature 441:1103-1108doi:10.1038/nature04789