As reported in a previous blog, the issue of New Scientist dated 27 February 2008 carried a major article on intermediate or transitional forms in nature. It was 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 second blog item, we consider Professor Prothero’s suggestion that:
Another key transition in animal evolution was the appearance of the vertebrates. For more than a century, evidence has been accumulating from anatomy and embryology that the Chordata phylum (which includes the vertebrates) evolved from the echinoderms - sea urchins, starfish and their kin. This has now been corroborated by molecular biology. We also have an array of fossils and living organisms to tell the story of the transition.
Professor Prothero is dogmatic. Chordates (and therefore vertebrates) have evolved from echinoderm (spiny skin) ancestors. Although sea urchins and starfish fossils (and their kin) do not appear in Cambrian strata, there are some strange and extinct echinoderms. These include Edrioasteroids, eocrinoids and helicoplacoids.
In 2004, Shu and his team described another group of early Cambrian echinoderms, namely, the vetulocystids. In the same issue of Nature, Andrew Smith, Merit Researcher at the Natural History Museum in London, wrote a review of the Shu paper entitled “Echinoderm Roots”. The full article is available here. On the second page of this review, we see a figure of the main deuterostome groups.
Deuterostomes comprise a super phylum which encompasses the echinoderms and the chordates (including the vertebrates). You will notice from this diagram that Professor Prothero’s suggestion in the New Scientist that “the Chordata phylum (which includes the vertebrates) evolved from the echinoderms” is without any scientific foundation. Furthermore, Andrew Smith states in this Nature article:
There is now direct fossil evidence that all of the major deuterostome groups were established by about 520 million years ago. Fossil vertebrates (yunnanozoans), tunicates and both asymmetric and radiate echinoderms (homalozoans, helicoplacoids) have all now been discovered in early Cambrian deposits. Nevertheless Professor Prothero is insistent, chordates have evolved from echinoderms. Although there is no evidence for this in the fossil record, he suggests possible intermediate forms. These include hemichordates (e.g. acorn worms and filter feeding pterobranchs) and more particularly tunicates. He states categorically:
Next up are the sea squirts, or tunicates. Though adult sea squirts are similar to pterobranchs, the larvae look much like primitive fish, with a muscular tail supported by a "backbone" of cartilage, the notochord - the defining feature of the chordates.
Although this is somewhat confusing, the Professor states correctly that tunicates are classified as chordates since they possess a notochord (cartilaginous rod) but only in their juvenile form.
Chen reported the discovery of eight specimens of an early Cambrian fossil tunicate Shankouclava near Kunming (South China). The full paper is available here. Again the fossil record clearly indicates that there were tunicates in the early Cambrian. Accordingly, it is not possible to conclude on the basis of the actual evidence that tunicates have evolved from echinoderms or from anything else. It is simply inferred from the general appearance of extant forms of these phyla. Maybe this is an alternative updated example of "phylogeny recapitulates ontogeny" that was originally proposed by Ernst Haeckel on the basis of his dubious embryonic diagrams.
Nevertheless, appearances can be very deceptive. We recommend looking up an image of a tunicate larva and considering whether Professor Prothero’s suggestion that they “look like primitive fish” has any basis in reality. Nevertheless, he does not give up:
The transitional sequence continues with a group of obscure invertebrates called the lancelets. These resemble [emphasis added] tunicate larvae, and probably evolved from a tunicate-like creature through "neoteny" - retention of juvenile features in adulthood. With a notochord, muscular tail, gill slits, a digestive tract along the belly and many other chordate features, lancelets are the most fish-like invertebrates known. They have been around since the Cambrian: we have a number of good lancelet fossils such as Pikaia from the Burgess Shale and similar fossils from Chengjiang.
What is certain is that these fossil lancelets appear very similar to the modern and not so obscure amphioxus (a living example of a cephalochordate) which is alive and very well on planet Earth. The amphioxus is extremely common in shallow sandy environments such as Discovery Bay in Jamaica where up to five thousand individuals per square metre of sand have been reported. Amphioxus is commercially harvested and eaten by humans and domestic animals in some parts of Asia.
A question that is never answered is this: If lancelets are intermediate transitional forms leading to the evolution of vertebrates, why do they appear essentially unchanged after 500 to 600 million years of evolutionary time? In addition, when and how do lancelets move away from being lancelets to become something other? Again, the fossil record provides no help in answering these questions because representative fossils of all the particular groups are found in the same strata.
Needless to say, of particular interest are the ancient fish fossils that have been found in the Cambrian. For example, in 1999, Shu and his group reported the finding of two species of ancient jawless fish, namely, Haikouichthys ercaicunensis and Myllokunmingia fengjiaoa. A report of this discovery is found on the BBC website In this article, Professor Simon Conway Morris (University of Cambridge) is quoted as saying:
Until now, the early history of the fish has been extremely sporadic and sometimes difficult to interpret. This discovery shows that fish evolved much earlier than was thought. It indicates also that the rates of evolution in the oceans during the Cambrian period must have been exceptionally fast. Not only do we see the appearance of the fish, but also a whole range of different animal types.Also in 1999, Chen et al. reported the finding of another early Cambrian chordate which they called Haikouella lanceolata. In their paper, they state that Haikouella has evidence of:
a heart, ventral and dorsal aorta, an anterior branchial arterial, gill filaments, a caudal projection, a neural cord with a relatively large brain, a head with possible lateral eyes, and a ventrally situated buccal cavity with short tentacles. These findings indicate that Haikouella probably represents a very early craniate-like chordate that lived near the beginning of the Cambrian period during the main burst of the Cambrian explosion.
More recently, Shu reported further studies made possible by the discovery of numerous specimens of the early Cambrian vertebrate Haikouichthys ercaicunensis. The paper presents further evidence that Haikouichthys was a fossil agnathan.
Agnatha are jawless fish and are represented today by hagfish and lampreys. Professor Prothero admits to the presence of these ancient vertebrates in the Cambrian:
Cambrian rocks in China have also yielded fossils of the earliest-known vertebrates, the soft-bodied jawless fish Haikouella, Haikouichthys, and Myllokunmingia. These creatures did not yet have a hard bony skeleton, but have all the other features of jawless fish supported by a skeleton of cartilage.But then he says something quite remarkable and illuminating:
Placed in sequence, the acorn worms, tunicates, lancelets and soft-bodied jawless fish show the complete set of steps needed to evolve a vertebrate from an invertebrate ancestor.
We suggest that this very superficial assessment is circular reasoning. The sequence chosen is based on the assumption that the Darwinian hypothesis is true. The resulting sequence is then used as proof of the assumption. Furthermore, the suggestion by Professor Prothero that molecular biology corroborates his interpretation is similarly without support in the academic literature. For example, Frederick Delsuc and his co-workers published a paper in Nature in 2006. It’s worth reading the abstract!
So recent advances in molecular biology do not corroborate Professor Prothero’s imagined sequence and his is only one of many that could be constructed. It all sounds very fishy to me!