Monday, July 16, 2007
Rearranging the evolutionary tree
Phylogeny is the study of the relatedness of species. How close are we to mice? How closely related are birds and reptiles? In the past, this has been figured on a morphological basis - that is, how closely physical features match. However, DNA analysis has in many cases turned on its head accepted wisdoms.
From the New Scientist, 16 June 2007 (Back to their roots):
"If you want to know how all living things are related, don't bother looking in any textbook that's more than a few years old."
But it's not only genetic analysis that has fostered the redesign of evolutionary maps. There is also the tool of molecular analysis, to develop a "molecular fingerprint" of different types of cells. This identifies a combination of characteristics that describes how molecules control a cell's genes, and what molecular signals a cell transmits and receives.
One of the new insights is a breakdown in the intuitive supposition that evolution involves a uniform move to increasing complexity. Plenty of examples have emerged to show organisms shedding characteristics that are no longer necessary. Whales shedding limbs is one example.
Parasitism provides other examples: loss of complexity can happen when motility is no longer needed. This is said to happen with a barnacle called Sacculina, which became parasitic (on crabs) at the larval stage, and so shed many characteristics of crustacea.
One area where this plays out is the control system. An invertebrate species called Platynereis dumerilii has a "neural net" type system, as opposed to a more sophisticated Central Nervous system. However, Detlev Arendt in Heidelberg claims through molecular fingerprinting that this species has more in common with vertebrates that descended from a putative common ancestor of bilaterals species called Urbilateria.
So... latest is not always best, and not always more complex than anything that has gone before. There is no evidence Homo sapiens have shed any characteristics. Yet... our brains are smaller than Neanderthals, which co-existed with humans, yet died out.