Anthropoid Origins

[CDarwin]

Another primate paleontology thread. I know everyone is excited.

 

This is a debate I've been lately reacquainted with reading Chris Beard's Hunt for the Dawn Monkey. I was wondering if anyone had read the book and/or had any particular opinions to offer on the origins of the Anthropoidea, the suborder of Primates consisting of the monkeys, apes, and humans. I'll attempt to summarize the debate if I might:

 

There are four competing theories that posit which group of the Eocene primates the Anthropoids descended from:

 

The Adapoid Theory: Believes anthropoids descended from lemur-like adapoids.

 

The Omomyoid Theory: Believes anthropoids descended from tarsier-like omomyoids.

 

The Tarsier Theory: Believes anthropoids descended from tarsiers themselves.

 

The 'Ghost Lineage' Theory: Believes anthropoids descended from some ancient and unknown group of primates in the Eocene that are uniquely related to the living anthropoids. Chris Beard supports this idea based on A) his view that the omomyoids are uniquely related to the tarsiers and B) his discovery of ancient alleged anthropoids in Asia, Eosimias.

 

Beard's notions also conflict with traditional ideas about the temporal and geographic origins of the Anthropoidea. All previous early anthropoids have come from North Africa, and with the possible exception of Algeripithecus, they haven't been older than about 35 million years.

 

So Beard believes the first anthropoids originated in Asia in the early to mid Eocene (up to 55 mya) and that they are an ancient lineage that originated after the Adapoidae and Omomyoidea had gone their own evolutionary ways. A graphical representation. Opinions?

[lucaspa]

Multiple competing hypotheses with insufficient data to decide between them. My opinion? Wait for more data.

[CDarwin]

Multiple competing hypotheses with insufficient data to decide between them. My opinion? Wait for more data.

 

Definitely fair. If it is a real anthropoid, though, Eosimias does seem suggestive. There's also the molecular data which I didn't touch on before. Apparently tarsiers and anthropoids diverged 55 million years ago, based on amino acid sequences. The weight of the evidence seems to lean toward Beard's suggestions.

[geoguy]

Multiple competing hypotheses with insufficient data to decide between them. My opinion? Wait for more data.

 

Agreed. I run into this all the time in less controversial evolutionary lines (invertebrates such as orders of brachiopods, rugose corals, etc.). We work with a richer fossil record but even still there are many unknowns.

 

It might be fun to speculate but speculation on human evolution is dependent on what book we've read last and no hands on experience. We aren't inmmersed in the primate evolution sciences and don't know which research, individuals, etc. have the most credibility within a profession. Primate evolution is educated speculation based on educated speculation...often 'a tooth begatting a tooth'.

[lucaspa]

Definitely fair. If it is a real anthropoid, though, Eosimias does seem suggestive. There's also the molecular data which I didn't touch on before. Apparently tarsiers and anthropoids diverged 55 million years ago, based on amino acid sequences. The weight of the evidence seems to lean toward Beard's suggestions.

 

1. That's a VERY big "if" there. The webpage you referenced states "In some aspects of its cranial morphology, Eosimias exhibits affinities with the omomyoids." That would lend support to the Onomyoid Theory, not Beard's.

 

2.The molecular data also fits the Onomyoid or Tarsier theory, too, doesn't it? Wait a minute, the molecular data actually is against Beard, isn't it? After all, Beard states that "anthropoids descended from some ancient and unknown group of primates in the Eocene that are uniquely related to the living anthropoids." If that were the case, then there would be no diverging from the tarsiers because the lineage is unique, not tarsiers. Therefore the molecular data should place anthropoids equidistant from tarsiers and all other known mammalian taxa. If Beard's hypothesis were correct.

 

The webpage didn't give an age for Eosimias. What is the supposed age of the fossil?

 

It might be fun to speculate but speculation on human evolution is dependent on what book we've read last and no hands on experience. We aren't inmmersed in the primate evolution sciences and don't know which research, individuals, etc. have the most credibility within a profession. Primate evolution is educated speculation based on educated speculation...often 'a tooth begatting a tooth'.

 

First, let's remember we aren't looking in this thread at human evolution, but considerably further back: the origin of the anthropoids. If the molecular data is correct (and it usually is), we are 48 million years before the split of chimps and hominids.

 

Second, since we don't have the hands-on experience, yes, we are confined to 1) speculation and 2) waiting for the scientific consensus to emerge. That's why I thought it was way too early to say Beard's ideas are more supported than the other ones. CDarwin, learn to be patient. Don't cheerlead a hypothesis; wait for the data. There is no "weight of data" yet.

 

Third, when we get to the hominid line we run into problems of secondary motivation among the scientists. Every paleontologist wants to either 1) have his fossils be in the direct ancestor-descendent line to humans or 2) be a new species. Those are the sexy claims. I can see this in the discussion of the 3 species discovered near the chimp-hominid split; all the investigators are sure the species they discovered are on the hominid side of the split. I also see this when new hominid fossils are discovered, particularly H. antecessori and A. garhi. Both, as described, sound like transitional individuals to me, but both are classed as completely separate species.

[geoguy]

1. That's a VERY big "if" there. The webpage you referenced states "In some aspects of its cranial morphology, Eosimias exhibits affinities with the omomyoids." That would lend support to the Onomyoid Theory, not Beard's.

 

2.The molecular data also fits the Onomyoid or Tarsier theory, too, doesn't it? Wait a minute, the molecular data actually is against Beard, isn't it? After all, Beard states that "anthropoids descended from some ancient and unknown group of primates in the Eocene that are uniquely related to the living anthropoids." If that were the case, then there would be no diverging from the tarsiers because the lineage is unique, not tarsiers. Therefore the molecular data should place anthropoids equidistant from tarsiers and all other known mammalian taxa. If Beard's hypothesis were correct.

 

The webpage didn't give an age for Eosimias. What is the supposed age of the fossil?

 

 

 

First, let's remember we aren't looking in this thread at human evolution, but considerably further back: the origin of the anthropoids. If the molecular data is correct (and it usually is), we are 48 million years before the split of chimps and hominids.

 

Second, since we don't have the hands-on experience, yes, we are confined to 1) speculation and 2) waiting for the scientific consensus to emerge. That's why I thought it was way too early to say Beard's ideas are more supported than the other ones. CDarwin, learn to be patient. Don't cheerlead a hypothesis; wait for the data. There is no "weight of data" yet.

 

Third, when we get to the hominid line we run into problems of secondary motivation among the scientists. Every paleontologist wants to either 1) have his fossils be in the direct ancestor-descendent line to humans or 2) be a new species. Those are the sexy claims. I can see this in the discussion of the 3 species discovered near the chimp-hominid split; all the investigators are sure the species they discovered are on the hominid side of the split. I also see this when new hominid fossils are discovered, particularly H. antecessori and A. garhi. Both, as described, sound like transitional individuals to me, but both are classed as completely separate species.

 

That's why I enjoy being in the field of paleozoic invertebrates, Nobody is after a 'gold star' in the media and rarely is anyone's knickers in a knot if further research questions previous conclusions.

[CDarwin]

1. That's a VERY big "if" there. The webpage you referenced states "In some aspects of its cranial morphology, Eosimias exhibits affinities with the omomyoids." That would lend support to the Onomyoid Theory, not Beard's.[/Quote]

 

The characters Eosimias shares with omomyoids are primitive, though, like the paraconid on the lower molars and the arterial configuration in the skull. It's not impossible that the common ancestor of omomyoids and anthropoids might have been similar, even omomyoid-like. The Omomyoid Theory proper posits a more recent ancestry from advanced omomyoids (the European Cercomiines usually), though.

 

I don't know if I can find a good internet cite for that, as the first thing that comes up when I google "omomyoid theory" is this thread. I'm going by Beard's book and Primate Adaptation and Evolution.

 

The derived anthropoid-like characters are what make Beard think that Eosimias in an anthropoid. The incisors in the lower jawEosimias are vertically oriented unlike those of most omomyoids and adapoids. The canines are large and blade-like completely unlike those of omomyoids and like those of adapoids and anthropoids. The "first" (really the second, as the first is lost) premolar has a single root, which is never the case in adapoids (the size is well below that of your average adapoid too, though this is also the case for anthropoids). Even more suggestive, the premolars sit diagonally in the jaw, with the distal root closer to the outside and the meisial root closer to the inside. This condition is never seen in omomyoids or adapoids but is in some anthropoids, like Saimiri.

 

2.The molecular data also fits the Onomyoid or Tarsier theory, too, doesn't it? [/Quote]

 

Those propose a recent ancestry for anthropoids (at the end of the Eocene). The molecular data is saying the last common ancestor lived 50-55 mya and the tarsier (and omomyoid if they're uniquely related) and human lineage have been evolving separately all that time. That's way beyond anything either those ideas propose.

 

Wait a minute, the molecular data actually is against Beard, isn't it? After all, Beard states that "anthropoids descended from some ancient and unknown group of primates in the Eocene that are uniquely related to the living anthropoids." If that were the case, then there would be no diverging from the tarsiers because the lineage is unique, not tarsiers. Therefore the molecular data should place anthropoids equidistant from tarsiers and all other known mammalian taxa.

 

Unique relative to the other primates and sharing a more recent, but still distant (Early Eocene-age), common ancestor with tarsiers than anything else. That's really what he's saying.

 

The webpage didn't give an age for Eosimias. What is the supposed age of the fossil?

 

About 45 million years. The age is probably the biggest problem as it's 5 to 10 million years younger than both the molecular date of divergence from tarsiers and perhaps the rather securely anthropoid but less securely dated Algeripithecus, and yet it's very primitive.

 

CDarwin, learn to be patient. Don't cheerlead a hypothesis; wait for the data. There is no "weight of data" yet.

 

That's true. I'm not totally on board with Beard but from what I've been exposed to he does seem to have the best explanation. That's what I meant.

[iNow]

I don't know if I can find a good internet cite for that, as the first thing that comes up when I google "omomyoid theory" is this thread. I'm going by Beard's book and Primate Adaptation and Evolution.

Have you looked at Beard's references section for a source or two?

 

http://books.google.com/books?hl=en&lr=&id=PgiGPYeVN0sC&oi=fnd&pg=PA55&dq=beard+omomyoids+&ots=oeDGRqhqx0&sig=3h0Hku8yn6m1V_KeQ8klwms6Pts#PPP1,M1