Freethought & Rationalism Archive

MrDarwin · 10-15-2003, 09:51 AM

Just came across a great article on craniate (chordates with heads) origins:

Mallatt & Chen 2003, Fossil sister group of craniates: Predicted and found. Journal of Morphology 258:1-31.

The paper provides detailed morphological descriptions of Haikouella, a recently discovered early Cambrian chordate (and with the closely related Yunnanozoan one of the two oldest known chordates), based on numerous exquisitely preserved fossils, and analyzes the relationship of this taxon to other chordates. The authors conclude that Haikouella is the sister taxon of (and although they do not explicitly say so, possibly the direct ancestor of) all other craniates. Among the conclusions from the abstract (note in particular the proven predictive value of evolutionary hypotheses):

Quote:

Haikouella was compared to pre-craniates predicted by recent models of craniate evolution and was found to fit these predictions closely. Specifically, it fits Northcutt and Gans' prediction that the change from ciliary to muscular ventilation preceded the change from suspension feeding to predatory feeding; it fits Butler's claim that vision was the first craniate sense to start elaborating; it is consistent with the ideas of Donoghue and others about the ancestor of conodont craniates; and, most strikingly, it resembles Mallatt's prediction of the external appearance of the ancestral craniate head. By contrast, Haikouella does not fit the widespread belief that ancestral craniates resembled hagfishes, because it has no special hagfish characters. Overall, Haikouella agrees so closely with recent predictions about pre-craniates that we conclude that the difficult problem of craniate origins is nearly solved.

Godless Dave · 10-15-2003, 10:46 AM

That doesn't prove anything. They're still the things-with-heads kind. </creationist>

MrDarwin · 10-16-2003, 08:59 AM

This article got me thinking: we often present evolutionary scenarios like fish => amphibian => reptile => mammal; if we're ambitious we might be a little more specific about the particular group of fish, or amphibians, or reptiles, or mammals leading up to humans. But creationists want us to give them nothing less than an unbroken chain, species by species, from any one "kind" to another. But since they accept species of a genus as comprising a "kind" perhaps they'll let us get away with doing this at the genus level.

In this case there's a good chance that the genus Haikouella is our direct ancestor. It meets two important criteria: morphological and temporal. It is one of the earliest known chordates, has been identified as the sister taxon to all craniates (which is the position that an ancestral taxon would take in a cladistic analysis), and lived at the right time (it is one of the earliest known chordates and predates any other known fossils that are likely to be on a direct line to vertebrates).

So here's my idea: how far could we get with a genus-by-genus lineage, going back from humans (or any other living species, for that matter) all the way to the Cambrian explosion (and maybe even beyond)? What are the likely candidates for the known fossil genera of fish, amphibians, reptiles, and mammals that are our direct ancestors, how strong is the evidence for these candidates, how many of the links in the chain are actually missing, and can we predict what to expect if those "missing links" are eventually found, as was predicted with Haikouella?

gallo · 10-16-2003, 01:41 PM

Not sure that would work. Before Homo there was Australopithicus. Before that there was Pan and then Gorilla? But that's not right. Chimps and gorillas are modern species and our common ancestor with them wouldn't have been in the modern genra.

MrDarwin · 10-17-2003, 06:37 AM

Pan fails the temporal test: it doesn't occur in the fossil record before the next most recent ancestor of humans (Australopithecus). It may fail the morphological test as well. Are there any fossil candidates that occur at the right time?

Although it does get me wondering, if we did find good fossils of the common ancestor of Pan and Australopithecus (or if the common ancestor were still living), how would it be classified? As Pan? As Australopithecus? Or as another genus entirely? Part of the problem is that we know so little about chimp evolution, like how far back Pan goes, and how many species there have been and how much they have changed since our common ancestor.

Getting back to the predictive power of evolutionary hypotheses, can we make any definite predictions about what we would expect to find (morphologically) in a common ancestor of the chimp and human lineages?

MrDarwin · 10-23-2003, 05:53 AM

More news, this time on a different fossil from Australia:

Museum Claims Fishlike Fossil Is Oldest

Our ancestor?

More coverage

lpetrich · 10-23-2003, 07:27 AM

That "fishlike fossil" is listed as 560 million years old, placing it in the Vendian. It had had a well-defined head and muscle blocks (somites), so it could simply ba an early chordate rather than specifically a vertebrate.

There's also an article about a 511-million-year-old crustacean fossil. Which beats out the Orsten fossils, the previous earliest examples.

MrDarwin · 10-23-2003, 07:49 AM

I thought some of the Burgess Shale critters were identified as crustaceans (stem group, if not crown group)?

N.Wells · 10-23-2003, 08:28 AM

Yes, Canadapsis means that crustaceans are represented in the Burgess Shale.

Valentine Pontifex · 10-23-2003, 01:12 PM

Quote:

Originally posted by MrDarwin

In this case there's a good chance that the genus Haikouella is our direct ancestor.

If by "good chance" you mean a small fraction.

The odds of it be directly ancestoral are very slim. There could have very easily been dozens of similiar genuses existing at the same time that we have not found yet and saying which one is the ancestor even if we had all the fossils would probably be difficult.

It is probably safe to say that it is the closest to the common ancestor of craniates yet discovered.

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10-15-2003, 10:46 AM	#2
Godless Dave Contributor Join Date: Feb 2002 Location: With 10,000 lakes who needs a coast? Posts: 10,762	That doesn't prove anything. They're still the things-with-heads kind. </creationist>

10-16-2003, 08:59 AM	#3
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	This article got me thinking: we often present evolutionary scenarios like fish => amphibian => reptile => mammal; if we're ambitious we might be a little more specific about the particular group of fish, or amphibians, or reptiles, or mammals leading up to humans. But creationists want us to give them nothing less than an unbroken chain, species by species, from any one "kind" to another. But since they accept species of a genus as comprising a "kind" perhaps they'll let us get away with doing this at the genus level. In this case there's a good chance that the genus Haikouella is our direct ancestor. It meets two important criteria: morphological and temporal. It is one of the earliest known chordates, has been identified as the sister taxon to all craniates (which is the position that an ancestral taxon would take in a cladistic analysis), and lived at the right time (it is one of the earliest known chordates and predates any other known fossils that are likely to be on a direct line to vertebrates). So here's my idea: how far could we get with a genus-by-genus lineage, going back from humans (or any other living species, for that matter) all the way to the Cambrian explosion (and maybe even beyond)? What are the likely candidates for the known fossil genera of fish, amphibians, reptiles, and mammals that are our direct ancestors, how strong is the evidence for these candidates, how many of the links in the chain are actually missing, and can we predict what to expect if those "missing links" are eventually found, as was predicted with Haikouella?

10-16-2003, 01:41 PM	#4
gallo Regular Member Join Date: Jun 2001 Location: Houston, TX, US Posts: 244	Not sure that would work. Before Homo there was Australopithicus. Before that there was Pan and then Gorilla? But that's not right. Chimps and gorillas are modern species and our common ancestor with them wouldn't have been in the modern genra.

10-17-2003, 06:37 AM	#5
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	Pan fails the temporal test: it doesn't occur in the fossil record before the next most recent ancestor of humans (Australopithecus). It may fail the morphological test as well. Are there any fossil candidates that occur at the right time? Although it does get me wondering, if we did find good fossils of the common ancestor of Pan and Australopithecus (or if the common ancestor were still living), how would it be classified? As Pan? As Australopithecus? Or as another genus entirely? Part of the problem is that we know so little about chimp evolution, like how far back Pan goes, and how many species there have been and how much they have changed since our common ancestor. Getting back to the predictive power of evolutionary hypotheses, can we make any definite predictions about what we would expect to find (morphologically) in a common ancestor of the chimp and human lineages?

10-23-2003, 05:53 AM	#6
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	More news, this time on a different fossil from Australia: Museum Claims Fishlike Fossil Is Oldest Our ancestor? More coverage

10-23-2003, 07:27 AM	#7
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	That "fishlike fossil" is listed as 560 million years old, placing it in the Vendian. It had had a well-defined head and muscle blocks (somites), so it could simply ba an early chordate rather than specifically a vertebrate. There's also an article about a 511-million-year-old crustacean fossil. Which beats out the Orsten fossils, the previous earliest examples.

10-23-2003, 07:49 AM	#8
MrDarwin Veteran Member Join Date: May 2001 Location: Washington, DC Posts: 4,140	I thought some of the Burgess Shale critters were identified as crustaceans (stem group, if not crown group)?

10-23-2003, 08:28 AM	#9
N.Wells Senior Member Join Date: Aug 2001 Location: Kent, Ohio Posts: 782	Yes, Canadapsis means that crustaceans are represented in the Burgess Shale.

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