Today's New York Times has this interesting article about some recent hominid fossil finds. Alas, it falls into the familiar trap of reporting every mundane find as if it is a scientific revolution:
Two fossils found in Kenya have shaken the human family tree, possibly rearranging major branches thought to be in a straight ancestral line to Homo sapiens.
Scientists who dated and analyzed the specimens -- a 1.44 million-year-old Homo habilis and a 1.55 million-year-old Homo erectus -- said their findings challenged the conventional view that these species evolved one after the other. Instead, they apparently lived side by side in eastern Africa for almost half a million years.
If this interpretation is correct, the early evolution of the genus Homo is left even more shrouded in mystery than before. It means that both habilis and erectus must have originated from a common ancestor between two million and three million years ago, a time when fossil hunters had drawn a virtual blank.
Fascinating! But I smell a version of the “If humans evolved from apes, why do we still have apes?” argument here.
The fact that erectus and habilis coexisted for a substantial period of time hardly implies that the former could not have evolved from the latter. It is certainly possible that both evolved from a common ancestor, but these two fossils hardly force such a conclusion. There is nothing implausible in suggesting that a species managed to coexist with one of its evolutionary descendants. Furthermore, I find this statement suggestive:
Although the findings do not change the relationship of Homo erectus as a direct ancestor of Homo sapiens, scientists said, the surprisingly diminutive erectus skull implies that this species was not as humanlike as once thought.
Based on this it sounds like the earliest erectus fossils were more ape-like (and more habilis-like) than the erectus fossils that came after them. That seems perfectly in accord with the idea that erectus is a descendant of habilis.
The article goes on to play up this idea that the fossils challenge “linear” evolution:
In any case, Dr. Leakey said, “Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis.”
Dr. Spoor, speaking by satellite phone from a field site near Lake Turkana, said the evidence clearly contradicted previous ideas of human evolution “as one strong, single line from early to us.” The new findings, he added, support the revised interpretations of “a lot of bushiness and experimentation in the fossil record,” rather than a more linear succession of species.
I have already explained why I am unconvinced by Dr. Leakey's argument. Dr. Spoor, on the other hand, seems to be exaggerating the significance of these finds. The idea of a simple linear progression of species leading from ape to man was discarded decades ago. I don't think you will find many paleontologists who are suprised to hear that the path of human evolution was more complex than the standard popular misconceptions.
We already know that starting roughly six to seven million years ago and proceding to the present there were a very large number of hominid species existing on various African plains. The fossils we have show a clear progression from the mostly ape-like oldest fossils through increasingly more human-like fossils, culminating in modern Homo sapiens roughly 100,000 to 200,000 years ago. No doubt the fossils we have represent only a fraction of all such species that actually existed. So there is an inherent implausibility in the idea that the ones we have just happen to be the ones that are on a direct ancestral line linking ancient Australopithecines to modern humans. From fossils alone it is essentially impossible to distinguish between a direct ancestor and a closely related side-branch. That notwithstanding, I fail to see anything in this article that requires a change in our thinking on human evolution.
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Actually, Tim White at Berkeley has been arguing for quite some time that the "bushiness" of the hominid tree has been greatly exaggerated, and that many fossils fall along the same track. Not that he's won the day, of course.
I forgot to mention: John Hawks had a write-up on White last year: http://www.johnhawks.net/weblog/fossils/anamensis/asa_issie_white_2006.w
The Times phrase "linear progression" strikes me that this was a science writer with a very poor understanding of evolution. Is it any wonder that the public is confused when the people responsible for conveying this information don't understand it themselves.
Unfortunately most of the other articles on this story are just as bad. The Wall Street Journal highlights this discovery as "Fossils Prompt New Evolution Theory" and writes that it is "creating messy kinks in the iconic straight line of human evolution . . . calling into question the evolution of our ancestors."
sigh.
In fact, it looks like the BBC wins again for their thorough analysis. Quoting Spoor they write that:
"'It's always possible that Homo habilis lived, let's say, 2.5 million years ago and then in another part of Africa, away from the Turkana basin, an isolated population evolved into Homo erectus.'
After a sufficient amount of time to allow both species to develop different adaptations and lifestyles, Homo erectus could have then found its way to the Turkana basin.
With separate "ecological niches", both species could co-exist without direct competition for resources."
The authors were clearly far more nuanced in their presentation than the American newspapers cared to report. Why am I not surprised?
From your description it sounds as though the writer should have spent some time at the human origins display at the American Museum of Natural History. I spent a little bit of time there on my visit to New York in the spring. The one thing that impressed me was the complexity of our origins. Here is a link to their web site: http://www.amnh.org/education/resources/rfl/web/hhoguide/tree.html.
When I logged onto the internet, the first thing that I saw was a news article splashed across my screen about the theory of evolution being "questioned" because of these hominid discoveries being found...blah ...blah. I think that the average person imagines evolution as this "march of progress" that species evolve enmass from one type of creature into another. The bushiness of evolution is a pretty simple concept to grasp. Why do so few people seem to understand it?
It gets far worse than the NY Times falling into the trap of seeing every mundane find as revolutionary. Some global warming deniers are now saying that if the scientists were wrong about this, they are also wrong about global warming.
To spare anyone interested in actually going to such a site, I did quote (and debunk) their bogus arguments at Liberal Values:
http://liberalvaluesblog.com/?p=1979
If anything, finding the most probable bushiness would increase our trust in both theory and especially findings.
Susan, I think there are several mechanisms at force here. AFAIK historically the idea of a linear and "progressive" development has lingered longest within anthropology and related areas. Humans, their societies and their immediate ancestors have been seen as "special" for no or little reason.
Did anyone notice that the article has been edited over the last few hours? When I first read it about 3 hours ago, in the section of the article that discussed the small skull size of the erectus specimen it contained this sentence:
Sexual dimorphism is considered a primitive trait in evolution.
And then there was a bit of how gorillas and early hominids like erectus show sexual dimorphism but chimps and modern humans have it to a lesser degree. This gave the impression that the reporter was still thinking in terms of evolution as "ladder of progress", drawing a connection that since gorillas shared a common ancestor with humans more remotely in the past they are primitive and would naturally be sexually diamorphic.
That sentence has now been removed. Maybe all the criticism is helping!
I remembered I had saved a copy of the original article. I misquoted in my previous comment. Here is what it said about sexual diamorphism:
Such a characteristic is thought to be a primitive stage in evolution. In humans, males average about 15 percent larger than females, and the same is true for chimpanzees. Sexual dimorphism is much more striking in gorillas, and apparently also in erectus.
"not as humanlike" might be referring to the discussion of sexual dimorphism at the end of the article, where the writer makes some more misleading statements anout evolution. Like "Such a characteristic [sexual dimorphism] is thought to be a primitive stage in evolution." (check Gene Expression for a discussion of that).
I think the researchers' idea is that many of the large erectus discovered so far are male, and the new surprisingly small one is female. It will be interesting if they do manage to show that erectus was very sexually dimorphic. But I wonder if they have ruled out population variation, imagine if a future archaeologist would discover only the remains of a pygmy female and a Masai male?
Just a note: dont approach the realm of evolutionary systematics, phylogenetic systematics and the like with simple logic and common sense. For example, at the one hand, paleontologists usually argue about series of fossils showing linear and gradual change: remember the recent debate concerning the origin of the great white shark that was echoed also in the SB-community? At the other hand, for cladists after any split (cladogenesis) you have two new species, by definition. For example, assume that habilis and erectus split 3 million years ago and a paleontologist belonging to the cladistic school discovers a 3.22 million years old habilis-like skull. Then the old skull must receive a new name and will be reported by the paleontologist as a new species, even if that skull is identical with a 2.33 million years old habilis skull.
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First, I think some of the bloggers here are panicking over spilled milk. They're upset over a few botched headlines that most of the reading public will quickly forget. I hardly see creationists turning people en masse over this, and while I honestly hate to give creationists any credit for anything, they can pat themselves on the back for making people here so paranoid.
Second, this is the only blog pointing out that scientists themselves can be a source of misinformation. I know it is much easier taking the simple-minded approach and bashing only the media, forgetting that science reporting takes at least two parties. I checked out several stories about this, and in most cases, the bad science people are complaining about is backed-up with direct quotes from the lead author of this study.
I've always believed that science reporters should approach individual scientists with the same level of skepticism as they should approach politicians. (Please note I said "individual scientists.") Scientists have a tendency to exaggerate the importance of their own discoveries, not because they're dishonest, but because it is what they've dedicated their lives too. It's a natural tendency for people to play up their own contributions in the grand scheme of things.
That said, one of the things that really irks me about many science reporters is they don't seem to go out of their way to get a broad range of opinion for their stories -- so when they cover a new discovery, many times they only talk to the authors of the paper being published. Talking to a few other scientists in the field would help paint a more complete picture about how this or that discovery fits into the scheme of things. Although, granted, scientists are not the easiest people to reach when you're facing a deadline in a few hours.
Jason writes: The fact that erectus and habilis coexisted for a substantial period of time hardly implies that the former could not have evolved from the latter. It is certainly possible that both evolved from a common ancestor, but these two fossils hardly force such a conclusion. There is nothing implausible in suggesting that a species managed to coexist with one of its evolutionary descendants.
The argument, as I understand it (which is not very well), for why it is unlikely that a species and its immediate descendent could coexist is this:
(1) If two species fill the same ecological niche then competition will lead to the extinction of one or the other (at least locally---the "losing" species may continue to exist somewhere else).
(2) Soon after the formation of a new species, the original species and the descendant species will have largely overlapping niches.
(3) Therefore, if the two species are to coexist, they must both evolve so that they occupy distinct niches.
The idea (which I had never heard of before, but it sounds plausible to me) is that when speciation occurs, it's never a single new species branching off from the main trunk, it's a split of one branch into two new branches. If a mutation is introduced into a species, there are three possibilities: (1) The mutants are better at everything than the nonmutants, in which case the mutants will soon displace the nonmutants. (2) The mutants are worse at everything than the nonmutants, in which case the mutants will die out. (3) The mutants will be better at some things, but worse at other things. (I guess a fourth possibility is that the mutation doesn't make any difference, but in that case, the mutants wouldn't form a new species, I wouldn't think.) The only way for the mutants to form a new species that coexists with the original species is case (3).
So if the mutants are better at some things than the nonmutants, and worse at other things, then it seems plausible to me that the mutants would in time evolve to focus on what they are better at, and the nonmutants would evolve to focus on what they are better at. So the single species splits into two new species, rather than an orignal species together with a descendant species.
As I said, I don't actually know what I'm talking about, I'm just trying to make sense of Leakey's statement that Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis.