Snapshots of the science11 Mar 2007
The new Human Origins hall at the American Museum is the occasion for a big Newsweek story, with the tagline, "The New Science of Human Evolution". Author Sharon Begley isn't stingy with the prose:
Whether or not you believe the hand of God was guiding these changes, the discoveries are overturning longstanding ideas about how we became human.
Not that fossils are passé. New discoveries are pruning and reshaping humankind's family tree as radically as bonsai. The neat traditional model in which one species gave rise to another like Biblical "begats" has been replaced by a profusion of branches, representing species that lived at the same time as our direct ancestors but whose lines died out. It's like discovering that your great-great-grandfather was not an only child as you'd thought, but had a number of siblings who, for unknown reasons, left no descendants. New research also shows that "progress" and "human evolution" are only occasional partners. More than once in human prehistory, evolution created a modern trait such as a face without jutting, apelike brows and jaws, only to let it go extinct, before trying again a few million years later. Our species' travels through time proceeded in fits and starts, with long periods when "nothing much happened," punctuated by bursts of dizzying change, says paleontologist Ian Tattersall, co-curator of the American Museum's new hall.
It's a little sad to see the article organized around a 15-year-old storyline. No More Unilineal Evolution! Hey, if it's a "new science", why do we keep hearing from the same old people?
Still, there are some brain evolution subplots, and a few genes mentioned. Aside from the flowery analogies, Begley is a good writer and can capture the essence of most of these stories in a few lines. As an exercise, let's try to take those few lines and change one crucial word to find the weakness of each hypothesis. For each quote, I'll strike out a word in the article and add the correct word in brackets.
You dirty louse
For example, let's start where the article does, with the "body lice = no fur" story:
That fork in the louse's family tree, [Mark Stoneking] and colleagues at Germany's Max Planck Institute for Evolutionary Anthropology concluded, occurred no more than 114,000 years ago. Since new kinds of creatures tend to appear
when[correct word: after] a new habitat does, that's when human ancestors must have lost their body hair for good - and made up for it with clothing that, besides keeping them warm, provided a home for the newly evolved louse.
You see how easy that is? Yes, new species adapt to new niches, but there is no reason to think this happens immediately. For that matter, there is no reason to think that hominids lost their fur instantaneously.
And hey, if the theme of the article is that human evolution has lots of extinct branches, then why doesn't that apply to louse evolution? We just saw last week how complex the louse phylogeny has been in hominoids. Who says that the current body louse was the first to fill that niche?
Oh, savanna, don't you cry for me!
Here's a short one:
The apes that stayed in the forests hardly changed; they are the ancestors of today's chimps. Those that ventured into the newly formed habitat of
dry grasslands[correct phrase: open woodlands] had taken the first steps toward becoming human.
None of the earliest hominid sites are open savanna. All of them come from sites that preserve other woodland creatures.
By the way, my favorite quote in the whole thing comes here:
Instead, evolution played Mr. Potato Head, putting different combinations of features on ancient hominids then letting them vanish until a later species evolved them.
I just love that analogy! Forget "mosaic evolution". I'm calling it "Mr. Potato Head evolution" from now on.
My what small teeth you have
This part is a little confused:
And it helps explain why Lucy's kind were the way they were. Afarensis women and men stood three to five feet tall and weighed 60 to 100 pounds. They had
small[correct: big] teeth good for fruits and nuts, but not meat. (The available prey was[correct: competing predators were] enough to make one a confirmed vegetarian: hyenas the size of bears, saber-toothed cats and other mega-reptiles and raptors.) That suggests that early humans were more often prey than predators, says anthropologist Robert Sussman of Washington University, coauthor of the 2005 book "Man the Hunted." The evidence is as stark as the many[correct: two] fossil skulls containing holes made by big cats and [correct: one containing] talon marks from raptors.
Well, that's taphonomy for you. There is plenty of evidence for predation on ancient hominid bones, and a National Geographic News article from 2002 details work showing the contribution of felids. But only two skulls have holes that may have come from ancient cats (those would be SK 54 from Swartkrans and D2280 from Dmanisi). Only Taung has evidence of raptor damage.
Splitting straws on habiline brains
Dmanisi has left people pretty confused about what explains hominid dispersal from Africa. Some are groping for other hypotheses. Just check out this paragraph:
Erectus shows that brain size is too crude a measure of a species' talents. At Dmanisi, the brains range from 600 to 770 cubic centimeters, comparable to the more primitive habilis. But while erectus did not distinguish themselves in brain size,
brain structure is more telling[correct: nor does its brain structure provide any clues]. They were[correct: They were not] the first of our ancestors to have an asymmetric brain, as modern humans do; Australopithecus species do not[correct: did]. Asymmetry is a mark of increasing specialization and therefore complex cognitive ability[correct: of questionable value, since apes and australopithecines have asymmetries to varying extents]. Erectus used it to, among other things, discover and tame fire [add: apparently much later]. What they did not use it for is technology. Tools found with the Dmanisi fossils include cutting flakes, rock "cores" from which flakes were made and a chopper, all primitive even for their time[correct: like those made in Africa]. "The old idea that you needed a master's degree in stone tools to leave Africa is crazy," says Bernard Wood.
Wow, how confusing. The Dmanisi crania had H. habilis-sized brains. They're like KNM-ER 1470. So brain size isn't the key characteristic that allowed hominids to disperse from Africa. Nor is body size, since the Dmanisi hominids were relatively small. That's a genuinely interesting problem.
But asymmetry doesn't solve it. KNM-ER 1470, either Homo habilis or Homo rudolfensis depending on your taste in hominids, has a well-defined Broca's area on the left hemisphere, which I would say is the main informative aspect of asymmetry in fossil endocasts. Chimpanzee brains are asymmetrical in some respects, so "asymmetry" itself is an irrelevant criterion without some specific anatomical feature in mind. The thing that people used to think might be important was petalial asymmetry -- one hemisphere of the cortex shifted forward compared to the other. Early Homo endocranial surfaces show fairly strong petalial asymmetries, including KNM-ER 2598 and KNM-WT 15000. But some Australopithecus endocasts share a similar pattern of asymmetry with later hominids (Holloway and De La Costelareymondie 1982). We don't know how to interpret petalial asymmetry in functional terms, by the way. There appears to be some correlation with handedness, but it's not clear that hand preferences and petalial asymmetries evolved at the same time or for the same reason.
Somebody could write a really interesting story just out of the material in this one paragraph. Just not this story!
Out of Africa
The bottleneck scenario always seems like a hard one for journalists to get right. This article is no better than usual:
Peter Underhill, a molecular anthropologist at Stanford University, tracked 160 such changes in the Y's of 1,062 men from 21 populations across the world. Applying the molecular-clock technique, he concludes that the most recent common ancestor of
all men[correct: all Y chromosomes] alive today lived 89,000 years ago in Africa. The first modern humans-and therefore, unlike the earlier wave of Homo erectus into Asia a million years ago, the ancestors of everyone today-departed Africa about 66,000 years ago.
These pilgrims were strikingly few. From the amount of variation in Y chromosomes today, population geneticists infer how many individuals were in this "founder" population. The best estimate: 2,000 men. Assuming an equal number of women, only 4,000 brave souls
ventured forth from Africa[correct: were isolated from other humans for thousands of years inside Africa]. We are their descendants.
Hard to get straight: genetic drift takes a long time to fix a gene. We don't necessarily know the number of founders of the out-of-Africa population; what we do know is how many individuals the ancient African population must have had under the hypothesis of genetic drift.
Other genes might well have more recent common ancestors, who would also have been more recent common ancestors of all men. This is especially true if any genes were under selection.
People who see my meetings talk will appreciate the irony of that last sentence...
Holloway RL, De La Costelareymondie MC. 1982. Brain endocast asymmetry in pongids and hominids: some preliminary findings on the paleontology of cerebral dominance. Am J Phys Anthropol 58:101-110. doi:10.1002/ajpa.1330580111