The Difference Between Fish and Humans

 
Here's a Press Release from BBSRC [The difference between fish and humans: scientists answer century old developmental question]. What's BBSRC, you might ask? Here's the answer,

The Biotechnology and Biological Sciences Research Council is one of seven Research Councils sponsored through the UK Government’s Office of Science and Innovation and invests around £336 million per annum in the biosciences.

BBSRC sustains a world-class bioscience research community for the UK. Its mission is to fund internationally competitive research, provide training in the biosciences, foster opportunities for knowledge transfer and innovation and promote interaction with the public and other stakeholders on issues of scientific interest.

Pretty impressive, eh? With a mission like that you'd expect a world-class press release, right? Let's see how they do ...

Embryologists at UCL (University College London) have helped solve an evolutionary riddle that has been puzzling scientists for over a century. They have identified a key mechanism in the initial stages of an embryo’s development that helps differentiate more highly evolved species, including humans, from less evolved species, such as fish. The findings of the research, funded by the Biotechnology and Biological Research Council (BBSRC), were published online today by the journal Nature.

Undergraduate students in my university are taught that humans and fish share a common ancestor. They have both evolved for the same length of time from that common ancestor, therefore you cannot say that one is more evolved and one is less evolved. People who say that are demonstrating a fundamental misunderstanding of evolution.

Early on in development, the mass of undifferentiated cells that make up the embryo must take the first steps in deciding how to arrange themselves into component parts to eventually go on to form a fully developed body. This is a process known as ‘gastrulation’. During this stage, the cells group into three layers, the first is the ‘ectoderm’ which then in turn generates the ‘mesoderm’ and ‘endoderm’ layers. In higher vertebrates, such as mammals and birds, the mesoderm and endoderm are generated from an axis running through the centre of the embryo. However, in lower vertebrates, such as amphibians and fish, the two layers are generated around the edge of the embryo.

Using the terms "higher vertebrates" and "lower vertebrates" is just as bad as using "more evolved" and "less evolved."

Scientists have been speculating for over a century on the difference between the embryonic development of higher vertebrates and lower vertebrates, to help answer how the simple cell structure of an embryo goes on to form the various highly complex bodies of different species. Research leader Prof Claudio Stern explains: “This is a significant find as it is a clear difference between the embryonic development of more advanced species and less advanced species. It suggests that higher vertebrates must have developed this mechanism later on in the history of animal evolution.

Scientists who use terms like "more advanced" and "less advanced" to distinguish modern species are demonstrating their ignorance. They should not be publishing papers about evolution.

The article can be found on the Nature website [Voiculescu et al., 2007)]. Some of the authors are well-known experts with impeccable reputations in the field of development biology (e.g., Lewis Wolpert). Thus, it is surprising that the press release is so bad. What does the paper actually say?

The paper describes the process of gastrulation and formation of the primitive streak in chicken embryos. It present results that support the involvement of a particular signalling pathway in this process. The data is supported by supplemental movies [Stage XIII].

The authors go on to compare the chicken pattern of cell movement and differentiation to that in Xenopus, an amphibian. Birds and mammals are amniotes and amphibians and fish are not (= anamniotes). The paper ends with a single paragraph that mentions evolutionary implications.

We propose that local intercalation in the epiblast is responsible for positioning and shaping the primitive streak and can also explain the polonaise movements without the need for long-range gradients. Convergent extension of the axial mesoderm and neural plate in anamniotes is almost certainly conserved in amniotes, but our study reveals an additional, much earlier (pre-gastrula) cell intercalation, required for morphogenesis of the primitive streak independently of mesendoderm specification. This is apparently unique to amniotes and provides a possible answer to the classical question of how evolution converted the equatorial blastopore or shield of Anamnia into the radially oriented primitive streak of amniotes.

There's nothing wrong with this. It does not claim that amniotes are "higher" than fish or amphibians and it does not claim that fish have stopped evolving.

The disconnect between the emphasis in the paper and in the press release is disconcerting. The differences in the language used to describe evolution is very troubling. The wording of the press release perpetuates the false concept of a "ladder of life" and that's not a way to advance the education of the general public. The fact that the press release quotes the senior author making the same conceptual error suggests that the errors in the press release may not be the fault of science journalists in the media department—although in an ideal world they should have been able to correct the scientists!

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Voiculescu, O., Bertocchini, F., Wolpert, L., Keller, R.E. and Stern, C.D. (2007) The amniote primitive streak is defined by epithelial cell intercalation before gastrulation. Nature (advance online pubication: doi:10.1038/nature06211).