Sunday, July 1, 2012

A thought about the terms "living fossil" and the tree of life

I every now and then come across people referring to species as living fossils, lower, primitive or ancient species. I personally feel that these terms shall be used with caution. The lineages encompassing all organisms alive today (the year is 2012 at the time of writing if you find this blogpost in an archeological dig in the future) have evolved from a common ancestor some billion years ago. The thing with that all organisms, at least all known, on earth share a common ancestor is that all lineages you can imagine have evolved for the same amount of time. A coelacanth, often reffered to as a living fossil or ancient species, have evolved for just the same amount of time as us humans. The major differences however is that the only two extant coelacanth species are very similar in shape to their fossilized relatives. The reason for this is not that the extant species is the same as the ones in the fossils, no these coelacanths have continued to change however at a slower rate in some characteristics making them appear "old" in their look. The situation in sort of the same when it comes to chimeras, sharks and rays. They appear to have stood still during the course of time, however the reason is not that the species are old but rather a slower change in characters that are beneficial for the life of these fish. 

The term "lower" in front of any chosen lineage is also often seen in both the literature and mentioned in various presentations by scientists. What do they really mean with this? The thing is that many still for some reason regard humans as sort of the perfected endpoint of an evolutionary tree with earlier (lower-) branching species groups, clades, regarded as less advanced than later branching groups (higher) and hence are they called lower vertebrates for example. However the tree of life is not anthropocentric, humans are not the end of a line of perfected selection. Rather the tree of life is a tumbleweed of life with branches going out in all directions. All extant species, or rather all living individual organisms, including you, me and the tiny ants on your lawn, are the endpoints of countless branches in the "tumbleweed of life", hence no life form is higher or lower, we are all at the same time point in the era of life on this planet.