Parsimony and Peripatric Speciation

[CDarwin]

"Finds test human origins theory" From the BBC Science and Nature. I don't have access to Nature to read the real article, so I'm just going by the BBC.

 

Here's the New Scientist article that I can only read the first paragraph of.

 

A Professor Spoor, quoted in the article, appears to feel that because of this discovery of Homo erectus and Homo habilis living at the same time, "[The idea that Homo erectus descended from an isolated populuation of Homo habilis] is a much more complex proposition [than peripatric speciation], the easiest way to interpret these fossils is that there was an ancestral species that gave rise to both of them somewhere between two and three million years ago."

 

Now why is that? Peripatric speciation is where a small founder population is isolated from a larger population in a different environment and speciates rapidly. In the mean time the mother population continues to change at its same, slow rate. That seems perfectly parsimonious. Is it really simper to think that H. habilis and H. erectus speciated at a roughly equal rate and time from an earlier species?

 

I realize I should note that the scenario that Spoor describes could itself be the result of peripatric speciation happening twice, but that would ruin the alliteration. I think you can tell what I'm getting at. I'm just not feeling particularly eloquent today.

 

EDIT: I remembered that I still have an ETSU account so I do have access to Nature, so I'm going to read that article.

[iNow]

"Finds test human origins theory" From the BBC Science and Nature. I don't have access to Nature to read the real article, so I'm just going by the BBC.

Here's another:

 

 

http://www.nyu.edu/public.affairs/releases/detail/1668

 

Two new fossils, described this week in the journal Nature, cast fresh light on a little understood and important period of human prehistory at the dawn of our own genus, Homo. The new fossils were discovered by the Koobi Fora Research Project, an international group of scientists directed by mother-daughter team Meave and Louise Leakey, and affiliated with the National Museums of Kenya (NMK). Human evolution over the last two million years is often portrayed as a linear succession of three species: Homo habilis to Homo erectus to ourselves, Homo sapiens. Of these, Homo erectus is commonly seen as the first human ancestor which is like us in many respects, but with a smaller brain. The new fossils are significant because both their relative geological ages and their physical attributes directly challenge these views about our human ancestry.

 

One of the two fossils, an upper jaw bone of Homo habilis (KNM-ER 42703), dates from 1.44 million years ago, which is more recent than previously known fossils of that species. This late-survivor shows that Homo habilis and Homo erectus lived side by side in eastern Africa for nearly half a million years.

“Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis,” explains Meave Leakey, one of the lead authors of the paper. Instead, both species must have had their origins between 2 and 3 million years ago, a time from which few human fossils are known. “The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition.”

 

The second fossil (KNM-ER 42700), found in the same region of northern Kenya, is an exquisitely preserved skull of Homo erectus, dated to about 1.55 million years ago. “What is truly striking about this fossil is its size,” says Fred Spoor, another lead author. “It is the smallest Homo erectus found thus far anywhere in the world.”

 

Read more of the article at the link above.

[CDarwin]

Here's another:

 

 

http://www.nyu.edu/public.affairs/releases/detail/1668

 

 

 

Read more of the article at the link above.

 

It mostly just recapitulates what the BBC article says, but thank you.

[iNow]

It mostly just recapitulates what the BBC article says, but thank you.

 

Well, technically, the BBC just recapitulated what the NYU site said, but you're welcome all the same.

[SkepticLance]

To CDarwin

 

When I first read the article, and the suggestion that the overlapping fossils 'proved' erectus did not evolve from habilis, my reaction was exactly the same as yours.

 

I see no reason why Homo habilis could not have split off a line that led to Homo erectus, while at the same time, elsewhere, continuing a line that remained habilis.

 

Of course, if the two lines once more moved to occupy the same territory, there would have to be a difference in ecological niche to minimise competition, or one would likely out compete the other, leading to the extinction of the 'weaker' line.

[lucaspa]

"Finds test human origins theory" From the BBC Science and Nature. I don't have access to Nature to read the real article, so I'm just going by the BBC.

 

I do have access to Nature and have read the article. I'll give you my conclusion first and then the reasons for it:

 

I speculate that Nature published the article in order to generate a discussion in the anthropological community about the hypothesis of erectus and habilis having a common ancestor instead of erectus evolving from habilis.

 

The article is VERY poor, ignoring a LOT of data that I, who know some but not all of the literature, am aware of. The article looks ONLY at the Dmansi fossils as intermediates between habilis and erectus and ignores whole series of fossils from Bed I at Olduvai.

 

Remember, they are only working with a calvaria, not a whole skull.

 

"Endocranial capacity, measured from computed tomography (CT)

scans and corrected for deformation, is estimated at 691 cm3. ... Although it is closer in overall size to H. habilis (Supplementary Note 1.1), we assign KNM-ER 42700 to H. erectus."

 

Notice this: the brain size is within the range for HABILIS, not erectus.

 

"Features suggesting

this attribution include frontal and parietal keeling, the mediolaterally

narrow temporomandibular joint, the distinct coronal and

sagittal orientation of the tympanic and petrous elements, respectively,

and a posterior midsagittal profile with a low occipital upper

scale and opisthocranion positioned close to lambda"

 

So this is why they assigned the calvaria to erectus.

 

BUT, there are features that contradict this. Notice how the authors try to handwave them away:

"2.1.1). Some characters often considered diagnostic

of this species (for example, a thick cranial vault and supraorbital

torus, and strong occipital angulation) are lacking in KNM-ER

42700, but scaling analyses of H. erectus and H. habilis crania suggest

that these features scale allometrically with cranial size13, and in any

event do not clearly differentiate the two taxa"

 

Really, "diagnostic characters" that "do not clearly differentiate the two taxa"? Come now, who are they kidding?

 

So, how to account for the small brain size of this erectus, which the authors admit:

"In external dimensions, KNM-ER 42700 is the smallest known

adult cranial vault attributed to H. erectus."

 

The authors state:

"This degree of variation may well imply that H. erectus showed marked sexual dimorphism, rather than the reduced levels that characterize the derived condition in H. sapiens (contra ref. 17)."

 

Now, they don't know whether the calvaria is from a male or female. However, a quick search around the web showed that, when the appropriate bones were present (hips for instance) the conclusion is that sexual dimorphism in H. erectus is small -- 20-30%. Far too small to account for the small cranial volume of KNM-ER 42700.

 

So, let me propose a different hypothesis: this is NOT an erectus calvaria. Rather, it is a calvaria from a transitional individual between habilis and erectus. After all, it does show a mosaic of habilis and erectus features. An individual that wandered into the Lake Turkana region from Olduvai, where the evolution of habilis to erectus was taking place.

[dichotomy]

Just how different do fossil bones have to be in human ancestry to designate them as a separate ‘taxa’? It seems it must be extraordinarily difficult to do. I say this just looking at the vast range of skeletal forms that exist today. I.e. Pigmies, Somalians, Watusi, Australian Aboriginal, Northern Europeans, Vietnamese, Kalahari bushman, The vast host of human irregularities – eg. Giants, dwarfs, down’s syndrome, microcephaly.

 

cheers.

[CDarwin]

Just how different do fossil bones have to be in human ancestry to designate them as a separate ‘taxa’? It seems it must be extraordinarily difficult to do. I say this just looking at the vast range of skeletal forms that exist today. I.e. Pigmies, Somalians, Watusi, Australian Aboriginal, Northern Europeans, Vietnamese, Kalahari bushman.[/Quote]

 

Those are all really pretty similar. Australians have big brow ridges and pygmies are smaller than average, but they all have big brains, gracile skulls, short faces, high foreheads, wimpy teeth, so on and so forth.

 

You are right though that distinguishing paleospecies is a pretty tough trick, especially in hominid evolution where everyone has an opinion. In fact, that wouldn't be a bad topic for conversation...

 

The vast host of human irregularities – eg. Giants, dwarfs, down’s syndrome, microcephaly.

 

You can spot all these pretty easily, with the exception of the last condition, which is the source of all the controversy over Homo floresiensis

[dichotomy]

You are right though that distinguishing paleospecies is a pretty tough trick, especially in hominid evolution where everyone has an opinion. In fact, that wouldn't be a bad topic for conversation...

 

I'll await this topic with eager eyes. I'm sure finding reasonable quantities of a paleospecies for starters, would aid in classifying them as distinct in the evolutionary ladder.

 

Cheers.

[CDarwin]

I'll await this topic with eager eyes. I'm sure finding reasonable quantities of a paleospecies for starters, would aid in classifying them as distinct in the evolutionary ladder.

 

Cheers.

 

Funnily enough that's not always the case. Sometimes you have so many fossils that an entire phenetic lineage is represented, and it's difficult if not impossible to define where one "species" starts and another begins.

 

The best example I can think, if it is a little obscure, would be the primates of the subfamily Anaptomorphinae (of the family Omomyidae) that lived in North America during the Eocene. In Wyoming there is a fossil record that documents step by step various changes in the jaws and teeth of the lineage related to a shift to a diet heavy on gums and tree exudates. There is all sorts of controversy as to the systematics.

[dichotomy]

Funnily enough that's not always the case. Sometimes you have so many fossils that an entire phenetic lineage is represented, and it's difficult if not impossible to define where one "species" starts and another begins.

 

 

Well, this makes complete sense to me. It's not as if humans change as suddenly as a tadpole turns into a frog. It's obviously an extremely slow, gradual process, that has 'bursts' of evolving speed and extinction when necessary, due to enviro pressures.

 

cheers.

[lucaspa]

Funnily enough that's not always the case. Sometimes you have so many fossils that an entire phenetic lineage is represented, and it's difficult if not impossible to define where one "species" starts and another begins.

 

Absolutely. One of my favorite quotes from the anthropology literature is:

 

"This primitive configuration of pongid and hominid traits has led the discoverers and describers of these early Australopithecines to assign them to a new species. If the first Australopithecines to be discovered is properly Australopithecus africanus, the early ones, they suggest, should be Australopithecus afarensis. Not al scholars agree. I have to confess that, although I have had the opportunity to handle both the Ethiopian and the South African material with which it is being compared, and although I agree with virtually all of what its describers say in regard to its tendency to be more primitive in a series of traits, I am not convinced that the differences are pronounced enough to warrant separate specific recognition. ...

"Our disagreement is merely a matter of the assignment of names. This is based on the judgement of the individual scholars and is a trivial matter, but it does point up an issue of fundamental significance. In an evolutionary continuum, change occurs more or less gradually through time. At the early and late ends of such change, everyone agrees that different names are justified, but when one form slowly transforms into another without break, the point where the change of name is to be applied is a completely arbitrary matter imposed by the namers for their convenience only - it is not something compelled by the data." C. Loring Bruce, "Humans in time and space." In Scientists Confront Creationism, edited by LR Godfrey, 1983, pp. 254-255.

 

Just how different do fossil bones have to be in human ancestry to designate them as a separate ‘taxa’?

 

See above. There are intuitive standards for designating species by morphology, BUT no specific ones.

 

It seems it must be extraordinarily difficult to do. I say this just looking at the vast range of skeletal forms that exist today. I.e. Pigmies, Somalians, Watusi, Australian Aboriginal, Northern Europeans, Vietnamese, Kalahari bushman,

 

In bones, these aren't as different as you think. One thing to remember is that many of the metrics are ratios, not absolute measurements of one variable. But ratios of 2 variables, such as length vs diameter of the femur. This eliminates many of the differences. Also, much of the time you are looking at angles, and again this is size independent. For instance, from the paper describing the calvaria:

"The mandibular fossa is deep but small, with well developed postglenoid and entoglenoid processes, the latter formed by the temporal bone, not the sphenoid. The tympanic is coronally oriented (tympanomedian angle2, 100°) whereas the petrous portion of the temporal is more sagittally oriented (petromedian angle2, 34°). The tympanic shows a faint petrous crest, which is well anterior (8mm) to the short, slender mastoid process. CT imaging reveals a relatively thin cranial vault, a small frontal sinus, a well developed sphenoid sinus, and a flexed cranial base (basion–sella–foramen caecum angle, 138°)."

 

You may not know the exact anatomical terms, but you can look to see that angles are being measured.

 

The vast host of human irregularities – eg. Giants, dwarfs, down’s syndrome, microcephaly.

 

Again, these are fairly easy to detect. They have multiple diagnostic symptoms that show up in the bones. The argument over H. floresiensis and microcephaly is being settled by looking at the long bones, not the skull. Recent reports show that the long bones are inconsistent with the individuals having microcephaly. That is, the long bones look normal, with the normal parameters (ratios and angles).

[dichotomy]

In an evolutionary continuum, change occurs more or less gradually through time. At the early and late ends of such change, everyone agrees that different names are justified, but when one form slowly transforms into another without break, the point where the change of name is to be applied is a completely arbitrary matter imposed by the namers for their convenience only - it is not something compelled by the data."

 

 

Well, if there are schools of thought on naming. I’d side with the school that doesn’t hand out names like, Neanderthal and Australopithecus africanus, to an obviously all too massive line of evolving human forms. It seems to me much more logical to simply identify specimens by the approximate time of their living activity and geographic location (e.g. specimen 54, approx’ 3 million years old, global coordinate location blah, blah, blah).

 

cheers

[SkepticLance]

One problem with viewing an apparent evolutionary continuum is that lots of fossils represent side branches - not the direct line of descent.

 

It is actually damn difficult, indeed impossible, to know if a fossil represents a direct ancestor, or a side branch - a cousin, so to speak.

 

The human line is especially difficult. We cannot be sure if Homo habilis or Homo erectus, or Australopithecus afarensis or any others are ancestors, or side branches of the hominid line.

[lucaspa]

Well, if there are schools of thought on naming. I’d side with the school that doesn’t hand out names like, Neanderthal and Australopithecus africanus, to an obviously all too massive line of evolving human forms. It seems to me much more logical to simply identify specimens by the approximate time of their living activity and geographic location (e.g. specimen 54, approx’ 3 million years old, global coordinate location blah, blah, blah).

 

That is done also. All fossils get a particular name. The calvaria and upper jaw in the study in this thread do have specific names. For instance, the upper jaw is KNM-ER 42703.

 

However, we are stuck with the Linnean classification. EVERY fossil gets put in a genus and species. There is no place in Linnean taxonomy to say "intermediate between H. habilis and H. erectus". So sometimes the species name is inaccurate and you have to ignore it and look for the reality of the description of the fossil.

 

Also, species are real entities. Once most species become adapted to a niche, they stay pretty much the same for until they go extinct. So there are a LOT of fossils that are definitely H. habilis (and are the same) and many that are H. erectus and are the same. It's when you find the transitionals that things get dicey. In the Nature paper, of course, the authors have a bias against the fossils being transitional because their hypothesis is that there should be different transitionals from a common ancestor of both erectus and habilis. Therefore the authors argue very strongly that these fossils are just like all the others of the species.

 

One problem with viewing an apparent evolutionary continuum is that lots of fossils represent side branches - not the direct line of descent.

 

It is actually damn difficult, indeed impossible, to know if a fossil represents a direct ancestor, or a side branch - a cousin, so to speak.

 

The human line is especially difficult. We cannot be sure if Homo habilis or Homo erectus, or Australopithecus afarensis or any others are ancestors, or side branches of the hominid line.

 

It's difficult, but not impossible. Especially for invertebrates, there are fossil records fine enough that you can trace the transitional individuals. In the hominid lineage, no one doubts that H. erectus is our immediate ancestor. The data in the form of transitional individuals is overwhelming. Up until this Nature paper, the data was clear that H. habilis was the ancestor of H. erectus. Again, there are transitional individuals and even in one place. As I noted, I've read descriptions of fossils from Bed I at Olduvai that have Habilis at bottom, then fossils with a perfect mixture of characteristics of habilis and erectus, and erectus at top. F. Clark Howell, Early Man, 1980, pg 81. Now, those fossils are from before 1980 and this paper only references the Dmansi find from 2001 (http://www.talkorigins.org/faqs/homs/d2700.html). I don't understand how the reviewers let the authors ignore the earlier data. Maybe they were unaware of it. But the Leakey's certainly are, since most of that earlier data is theirs!

 

There are also transitional individuals linking A. afarensis to H. habilis. That doesn't mean there aren't side branches. H. erectus in Asia is a side branch. So are neandertals and H. rudolfensis. So is the entire genus of Paranthropus. But until this paper the direct line back thru 2 species to A. afrarensis was pretty well established. And I think the data in the paper is really too weak to challenge the other evidence.