In this chapter, and in chapter 1, I emphasised that the history of life is like a tree, with a small number of branches at the beginning splitting repeatedly into numerous descendants. That is indeed the usual pattern, and that’s why we make phylogenies to illustrate evolutionary history. However, sometimes branches which were previously separate fuse. This happens a lot in prokaryotes, where individuals of different strains exchange genes, and the prokaryote phylogeny is difficult to reconstruct for this reason. It also happens occasionally in eukaryotes. For example, there is a Colombian butterfly species Heliconius heurippa which actually originated from the hybridization of two other species. Its phenotype is intermediate, its genome is a mixture, and the hybrids mate amongst themselves rather than with either of the parent species. To come into existence, then, some individuals of the two parent species must for some reason have mated together, producing enough butterflies that they could then start mating amongst themselves. To show that this is possible, Mavárez et al. (2006) created hybrids between the two parent species in the laboratory (you can do this by putting them into proximity and giving them no other choice about who to mate with). The hybrids produced looked exactly like Heliconius heurippa, and moreover preferred to mate with Heliconius heurippa than with either of the two parent species. This kind of event is probably reasonably rare in animals, though more common in plants, but it still means that sometimes the tree of life will look more like a thicket.

In May 2009, Franzen et al. (2009) published a description of a beautiful, fully preserved fossil primate from Germany. It dates from the Eocene, around 55 million years ago. There was a lot of media hype around the publication, of the ‘missing link’ type, which may or may not be justified. It’s not clear that it’s a direct ancestor of the haplorrhines, although it could be. It could be a primitive type of lemur, though it lacks some of the derived features of living lemurs such the tooth comb which they use for grooming themselves. Anyway, regardless of how its taxonomic status is eventually sorted out, it will help our understanding of primate evolution, and it is one of the most beautiful fossils I have ever seen. The paper includes stunning photographs and is freely available at:

http://www.plosone.org/article/info:doi/10.1371/journal.pone.0005723 [DOI: 10.1371/journal.pone.0005723]
Franzen, J.L. et al. (2009). Complete primate skeleton from the Middle Eocene of Messel in Germany: Morphology and paleobiology. Plos ONE 4: e5723.

Mavárez, J. et al (2006). Speciation by hybridization in Heliconius butterflies. Nature 441: 868-71 [DOI: 10.1038/nature04738]