Friday, March 03, 2006

The current tree of life


Carl Zimmer's blog, The Loom, features this image with better resolution. It comes from biologists at the European Molecular Biology Laboratory, who have published in Science the most thorough tree of life based on sequencing 31 universal genes selected from 191 species of animals, plants, fungi, protozoans, bacteria, and archaea.

Zimmer explains the diagram:

Here's a quick tour of the tree. Start at middle of the circle. The central point represents the last common ancestor of all living things on Earth. The tree sprouts three deep branches, which between them contain all the species the scientists studied. These deep branches first came to light in the 1970s, and are known as domains. We belong to the red domain of Eukaryota, along with plants, fungi, and protozoans. Bacteria (blue) and Archaea (green) make up the other two domains.

These lineages probably split very early in the history of life. Fossils of bacteria that look much like living bacteria turn up at least 3.4 billion years ago. Just a few lineages became multicellular much later, with some algae getting macroscopic about two billion years ago.

The length of the branches on this tree represent so-called genetic distance. The longer the branch, the more substitutions have accumulated in its genes. Since these genomes all come from living species, the branches all span the same period of time. The fact that some branches are long and some are short means that some lineages have evolved more than others. Many forces can stretch out genetic distance. A species may reproduce fast, or it may have a life that makes it prone to acquiring more mutations. The slash in the Bacteria branch represents a segment that the scientists left out to make the full tree easier to see.

It's amazing how small the animal kingdom is in the picture--if "speciesism" is a real problem, People for the Ethical Treatment of Animals (PETA) are apparently guilty of it by focusing only on animals.

2 comments:

amstar said...

Hi Jim,

This paper is a great contribution and it is good to see this tree getting some press because people are too often exposed to one like this but the dominance of bacteria is a function of the number of fully sequenced genomes in this domain.

I can't resist putting my critics hat on for a moment:

As impressive as this effort is, it is constrained by the limits of available data. The tree does not actually contain much of the true diversity present in either the bacterial or archaeal branches as so few organisms from either of these domains have been cultured, let alone sequenced. My guess is that if it could be representative of the true diversity (and kept constrained to the 360 degrees available in a circular tree) the bacteria would shrink a little as would the eukaryota and there would be a significant increase in the archaea. Forcing the tree topology into a circle can really distort things so I am not sure it is the best way to represent this data. Also, the relationship between the three domains is fairly controversial and the pattern shown in this tree is strongly influenced but the choice of genes used.

Of course there is always this tree to fall back on.

Andrew

Jim Lippard said...

Andrew:

Thanks for the comment, I appreciate the grains of critical salt to keep in mind!