Brussels, 02 Dec 2005
Scientists at the European Molecular Biology Laboratory (EMBL) in Heidelberg are working on a study that they say shows that a small marine worm has complex human-like genes. The discovery, recently published in the scientific journal Science, contradicts the classical theory that evolution to higher forms of life is linked to gene complexity gains.
The research provides evidence to suggest that the basic genetic programming of superior forms of life is as old as life itself. According to the EMBL scientists, the first animals had human-like complex genes, and, during evolution, human species retained characteristics of this very ancient ancestor that were lost in more quickly-evolving animals.
In order to establish what early animals were like, scientists usually look at their descendents. This is difficult when comparing distantly-related animals such as humans and flies. In these cases, it helps to look at living organisms that have preserved many features of their ancestors. The EMBL group, led by Detlev Arendt, has focused on a small marine worm known as the Platynereis dumerlii, and its fossil-age ancestor.
Until quite recently, such comparisons could only be made by looking at physical characteristics such as the structure of bones, teeth, or tissues. But DNA sequencing now permits scientists to make comparisons of the genetic code and read evolutionary history from it. An international consortium involving researchers from EMBL, the UK, France and the United States has thus sequenced a part of the Platynereis genome.
According to researcher Florian Raible, performer of most of the computer analyses: 'The fraction of Platynereis genes we have been able to look at tells a very clear story. The worm's genes are very similar to human genes. That's a much different picture than we've seen from the quickly-evolving species that have been studied so far.'
Dr Raible has also been involved in research that may put an end to a scientific controversy over evolution.
Genes hold the codes for the synthesis of proteins. However, in 1977 a new phenomenon was discovered: introns. Between the working or expressed segments of genes ('exons'), the genes of more evolved, multicellular plants and animals also contain extra bits of non-coding sequences of DNA, which have no apparent purpose. Introns almost never appear within prokaryotic cells, and are rare within single-celled eukaryotes. The number of introns in genes also varies greatly among animals: while humans have many introns in their genes, common animal models such as flies have fewer.
This evidence had led to the assumption among some scientists that, from an evolutionary perspective, the simpler (fly) genes would be more ancient. Simple organisms have no or few introns as these are added in the course of evolution. Now, the EMBL study has revealed the opposite: early animals already had a lot of introns, and it is quickly-evolving species, like insects, that have lost most of them.
'Human genes are typically more complex than those of flies,' explains the lab's principal investigator, Peer Bork. 'Classically studied species like flies have far fewer introns, so many scientists have believed that genes have become more complex over the course of evolution. There have already been speculations that this may not be true, but proof was missing. Now we have direct evidence that genes were already quite complex in the first animals, and many invertebrates have reduced part of this complexity.'
This discovery supports the 'introns-early', also called the exon theory of genes. The theory goes that exons were minigenes that at some stage, such as at the pre-cellular stage, would have functioned as genes do today. According to this theory, at a later stage in evolution, the minigenes were assembled to make whole genes and introns would have been the functionless pieces that held the exons together. All genes were built that way, and if bacteria and single-celled eukaryotes have none or very few introns, it is because they lost them in later evolutionary stages.
And not only are the introns there - the team also discovered that their positions within genes have been preserved over the last half a billion years.
'This gives us two independent measurements that tell the same story,' explains Dr Raible. 'Most introns are very old, and they haven't changed very much in slowly-evolving branches of life, such as vertebrates or annelid worms. This makes vertebrates into something like 'living fossils' in their own right.'
The scientists believe that the discovery that Platynereis also represents a slowly evolving branch of animal life has important implications for the study of humans. 'We've already learned an incredible amount about humans from studies of the fly,' Dr Arendt says. 'The marine worm might well give us an even better look at important conserved processes. Another thing that this has shown us is that evolution is not always about gain; the loss of complexity can equally be an important player in evolution,' he concludes.
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Vertebrate-type intron-rich genes in the marine annelid Platynereis dumerilii. F. Raible, K. Tessmar-Raible, K. Osoegawa, P. Wincker, C. Jubin, G. Balavoine, D. Ferrier, V. Benes, P. de Jong, J. Weissenbach, P. Bork and D. Arendt. Science, 25 November 2005