|What Missing Link?|
The issue of New Scientist dated 27 February 2008 carries a major article on intermediate or transitional forms in nature. It is written by Donald Prothero who is professor of geology at Occidental College in Los Angeles and lecturer in geobiology at the California Institute of Technology in Pasadena. His book “Evolution: What the fossils say and why it matters” is published by Columbia University Press.
In this article, Professor Prothero cites 10 examples to demonstrate the concept of a transitional form and to reinforce the idea that the fossil record provides evidence for the Darwinian hypothesis. The first of these examples are the velvet worms which belong to the phylum Onychophora, meaning 'claw-bearers'. At least eighty living species are known comprising two families, namely, the Peripatidae and the Peripatopsidae. They are found in tropical or sub-tropical regions of the Americas, Africa, South Eastern Asia and Australasia, which has the greatest variety.
The Onychophora first appear in the fossil record in the Cambrian Explosion. This geological event presents a significant problem for Darwinism since around half of the major animal phyla appear in the fossil record fully formed without any fossilised ancestors. This is how Richard Dawkins describes it in “The Blind Watchmaker” (page 229):
The Cambrian strata of rocks, vintage about 600 million years, are the oldest ones in which we find most of the major invertebrate groups. And we find many of them already in an advanced state of evolution, the very first time they appear. It is as though they were just planted there, without any evolutionary history.Although there have been more suggestions that the Onychophora are intermediate between annelids (segmented worms) and arthropods (a phylum including crustaceans and insects), according to Professor Prothero:
A classic example of a transitionary form links the arthropods to the lineage they split from in the Cambrian, namely, the nematode worms. These are the "velvet worms" or Onychophora. In many respects, the velvet worms resemble nematodes, but they also have key attributes of the arthropods - most notably segmented legs that end in hooked claws. They also have many other features found in arthropods but not nematodes, including an outer layer made of chitin, which they moult on a regular basis, antennae, compound eyes and arthropod-like mouthparts.
Unlike other scientists, Professor Prothero suggests that the velvet worms are derived from nematodes (roundworms) which are one of the most common phyla of animals, with more than 20,000 living species. Fossil nematodes are relatively rare but they do exist. The best examples of ancient nematodes have been found preserved in amber and dated from the Cenozoic period. Another example is a putative nematode (Captivonema cretacea) from the early Cretaceous. Whether nematode fossils will ever be found in Cambrian (or pre-Cambrian) strata remains to be seen and according to Simon Conway Morris:
To date, however, the fossil record throws no useful light on the origin of the nematodes. [Conway Morris S. (2004) Darwin’s Dilemma: The realities of the Cambrian Explosion Philosophical Transactions of the Royal Society. 361: 1069–1083].One of the reasons suggested for the absence of nematode fossils in Cambrian strata is their microscopic size and possession of soft body parts. Nevertheless, current research suggests that at least part of the Cambrian strata was laid down very rapidly [see Gabbott SE et al. (2008) Journal of the Geological Society; 165: 307-318]. This at least explains the amazing fossil preservation of myriads of soft bodied creatures.
So Professor Prothero’s suggestion that the Onychophora are transitional between nematodes and arthropods cannot be borne out by the actual evidence presented in the fossil record. It is based on the rather superficial assessment that “velvet worms resemble nematodes”. He concludes this section of his article by stating:
You could not ask for a better "missing link" between the nematodes and the arthropods, except it's not missing - we've known about velvet worms for over a century in both the living fauna and the fossil record.
Although this statement is somewhat misleading, at least the Professor highlights the fact that Onychophora might be considered “living fossils”. The ancient Cambrian species Aysheaia seems remarkably similar to the modern Peripatus.
In future blogs, we will consider some of Professor Prothero’s other examples.
Speculations on the chemical origins of life are almost universally covered in school curricula under ‘Evolution’, despite the questionable relevance of the topic for evolution, and its rather uncertain scientific basis.
Moore, A. (2008) Nature 453:31-32