h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I think it was actually the Pre-cambrian explosion.

I studied genetics for a while at university, and this was quoted as evidence for evolution! I think the theory went like this;

Higher animals evolved from segmented animals like annelid worms (eg. earthworms). In these worms each segment is more or less the same, the body forms a repeating pattern.

The pre-cambrian explosion occured with the evolution of regulatory genes that could control the development of a segment become one of a pattern. These genes (homeotic genes) activate or suppress other genes, and so have wide ranging effects. Each segment could be specialised, for example in the fruit fly (Melanogaster something?) has a maggot that is a visibly segmented worm-like grud. When it becomes a fly, some of the segments form the head, some the thorax, and some the abdomen. The fate of each segment is decided by the regulatory gene.

This can be demonstrated by genetic engineering. If the regulatory gene for the antenna is replaced with that for the leg, then a leg developes in place of the antenna. Called antennapedia (scientists love giving things latin names)

This was also given as the reason for why we look so different to chimpanzees, yet share something like 97% of their DNA. The bits that differ are mainly regulatory genes. It takes lots of genes to tell an animal how to grow hair, but only a few regulatory genes to tell the animal where to put it.

Once these regulatory genes evolved, a change (mutation) in these genes would have a great effect on the organism. This meant that lots of variation occurred in the population, and this lead to a faster rate of speciation (creation of new species).

...Or something, someone please correct me if I'm wrong.

D.

I haven't heard that explanation for it, but it seems to make sense.

Just look at a thing like throwbacks. One little mutation in a regulatory gene and a guy is born with hair all over his body. Or a horse born with toes like it's ancestors.

But because evolution usually happens with changes in the enviroment, you would expect great evolutionary changes when the enviroment changing rapidly. And it seems that the cambrian explosion happened in one of those cases, where Earth almost turned into a snowball. Also other great changes have shown to have happened because of such disasters.

Indeed Xanatic, I personally don't think these two theories need be mutually exclusive.

I've heard both cited as explanations for the Cambrian explosion. An increase in the number of transcription factor genes (HOX genes) will allow a rapid increase in cellular diversity and hence species diversity. If you combine this with a time shortly after a mass-extinction (the demise of the trilobites I believe) then you have nice juicy environments with little or no existing competition to exploit.

This of course may all be completely wrong, but who will ever know seeing as it happened about 4000 million years ago (or thereabouts).