Origins of the tree of life (re-post)

August 27, 2010
The chicken or egg blog family is on holiday in Germany during August, so I probably won’t have a chance to write any new posts.  To keep you all entertained, I’ll be re-posting some of my earlier (pre-Sciblogs) articles. This post is from August 2009.

Darwin is usually credited with being the first person to describe relationships among species as a tree.  I’ll admit I always thought this was the case, until this week when some discussion on an evolution email list I subscribe to enlightened me.

Darwin used this tree figure in The Origin of Species to illustrate his idea of “descent with modification”, with the branches representing the diversity of species all interconnected back through time.

Darwin’s tree of life – the only figure in The Origin of Species.

However, some 50 years earlier, in fact in the year of Darwin’s birth 1809, Jean-Baptiste Lamarck used a tree of sorts to depict evolution in his book Philosophie Zoologique.  Lamarck is best known for wrongly believing that evolution happened by the inheritance of acquired characteristics, but he should be given credit for being the first scientist to develop a theory of evolution – unfortunately he just had the mechanism wrong.

But ideas about trees were around even before Lamarck’s time.  Perhaps the earliest description of a tree of life comes from Russian naturalist Peter Simon Pallas, in 1766:

But the system of organic bodies is best of all represented by an image of a tree which immediately from the root would lead forth out of the most simple plants and animals a double, variously contiguous animal and vegetable trunk; the first of which would proceed from mollusks to fishes, with a large side branch of insects sent out between these, hence to amphibians and at the farthest tip it would sustain the quadrupeds, but below the quadrupeds it would put forth birds as an equally large side branch.

The earliest published tree diagram likely comes from 1801, when French botanist Augustin Augier published his Arbre Botanique, a detailed tree diagram complete with leaves that depicted his view of the relationships between members of the plant kingdom.

Arbre Botanique

Of course the theory of evolution has advanced somewhat since Darwin’s time and we now know that the idea of a tree of life is a little too simplistic.  As explained in a recent article in New Scientist (which was published with the outrageously inflammatory cover “Darwin was wrong” that no doubt excited a few creationists), some species relationships, particularly of the earliest organisms, are better described by a network rather than a tree, which acknowledges that hybridisation and horizontal gene transfer play a big role in evolution.

If you want to read more on the history of the tree of life, have a look at this article by David Archibald from San Diego State University.


An interview with Rebecca Cann of Mitochondrial Eve fame

June 1, 2010

In 1987, Rebecca Cann, Mark Stoneking and the late Allan Wilson published a paper in Nature showing that all human females can trace their lineage back to a single maternal ancestor (“mitochondrial Eve“) located in Africa.  In Plos Genetics this week there is an interesting interview with Rebecca Cann, where she talks about her own history and the research behind the mitochondrial Eve hypothesis.

In unearthing the genetic history of human populations, the recent pace of discovery has been so rapid that we can lose sight of the impact made by a single paper. In a 1987 Nature article, Rebecca Cann and her co-workers, Mark Stoneking and the late Allan Wilson, painstakingly analyzed mitochondrial DNA purified from placentas that had been collected from women of many different ancestral origins. By comparing the mitochondrial DNA variants to each other, the authors produced a phylogenetic tree that showed how human mitochondria are all related to each other and, by implication, how all living females, through whom mitochondria are transmitted, are descended from a single maternal ancestor. Not only that, they localized the root of the tree in Africa. The report left a wake, still rippling today, that stimulated not just geneticists and paleo-anthropologists, but the layperson as well, especially as the ancestor was quickly dubbed “Mitochondrial Eve.”

For the full interview, see Gitschier J (2010) All About Mitochondrial Eve: An Interview with Rebecca Cann. PLoS Genet 6(5): e1000959. doi:10.1371/journal.pgen.1000959


Origins of the tree of life

August 22, 2009
 Darwin is usually credited with being the first person to describe relationships among species as a tree.  I’ll admit I always thought this was the case, until this week when some discussion on an evolution email list I subscribe to enlightened me.

Darwin used this tree figure in The Origin of Species to illustrate his idea of “descent with modification”, with the branches representing the diversity of species all interconnected back through time. 

Darwin's tree of life - the only figure in The Origin of Species.

Darwin's tree of life - the only figure in The Origin of Species.

Read the rest of this entry »