Palaeontology and genetics in perfect harmony

It is easy for geneticists to feel superior about their methods: any organism’s genome can be fully sequenced in a matter of hours, providing a wealth of data that seem near-infinite in potential. In contrast, more traditional branches of natural history such as palaeontology can seem a lot of work for very little return. However, far from new techniques in one area leaving another discipline redundant, a recent paper in Science demonstrates how complementary both traditional and modern methods can be, and the startling results that can be produced by employing both together.

The authors investigated the diversification of mammals from a few basal ancestors to the roughly 5,400 species alive today.  They produced two important results: a phylogenetic tree of every family (the third most specific taxon, after species and order) in the mammals; and dates for the major radiations (splitting into different taxa) of basal mammals.  The tree was calculated by sequencing and aligning 26 DNA sequences (21 coding for genes, 5 non-coding) from every one of the insert mammalian families. Previous trees had grouped families based on morphological similarities, and so were vulnerable to mistakes or distortion caused by convergent evolution (when two distantly related species (or families) adapt to an environment in a similar way, producing similar traits and thus giving the illusion that they are closely related). Using DNA sequence data allowed the authors of this study to look beneath such misleading facades: even rapid, dramatic evolution will not mask the genetic similarities two families share in other areas of their genome.

However, DNA alone does not reveal how recently different families split apart from each other. Gene sequences reveal how families are grouped within a tree, and which branches connect one family to another, but other data are needed to tell us how long these branches are in terms of years. And this is where the fossil evidence comes in. By looking at the ages of fossils of the outgroups (distantly related species which are used to provide a sense of “scale” when constructing a phylogenetic tree), the authors were able to date the overall length (age) of the tree. With the branch lengths known, important moments in the evolution of the mammals could be given a concrete date.

This produced two main results. The first reveals more detail about the pattern of evolution before and after the Cretaceous-Palaeogene (K-Pg) extinction event (formerly known as the K-T extinction event, caused by asteroid hits 65 million years ago which led to the eventual demise of the non-avian dinosaurs, along with many other groups). A large increase in the formation of new orders was seen during the Cretaceous, before the asteroid hit; and most mammalian crown-group orders were already established by the end of the Cretaceous, before the asteroid hit. The authors credit the extinction event with “opening up of ecospace”: the crown-group order ancestors could then diversify into their current broad range of species. This has important implications for future palaeontological research: as the authors say, “we are unlikely to find crown rodents or primates in rocks that are much older than the latest Cretaceous”, thus helping to direct the hunt for fossils into the correct stratum.

The second result produced by this dated tree is a refutation of the “delayed rise of present-day mammals” theory: this states that mammalian diversity remained relatively constant after the K-Pg extinction event, and diversified during the Eocene, 10 million years later. The authors of the present study found no such Eocene increase, but instead more diversification soon after the K-Pg border, lending weight to the more traditionally held belief that the extinction of so many groups left the mammals with many empty ecological niches to fill.

What I like most about this paper is its demonstration of the great potential of combining such seemingly disparate fields: palaeontologists now have a more reliable phylogeny to aid in fossil location, identification and analysis; and evolutionary geneticists can place their sequence data in the context of the ecological conditions and events that may have driven the evolution of those sequences.

References

Bininda-Edmonds et al (2007) The delayed rise of present-day mammals.  Nature 446: 507-512

Meredith, R.W. et al (2011) Impacts of the Cretaceous Terrestrial Revolution and KPg Extinction on mammal diversification.  Science 334: 521-524

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