Begetting megabats

Taxonomists now regard the paramomyids as a sister group to the primate-like plesiadapoids, grouping them in the extinct order, the plesiadapiforms, which flourished in Europe and America from the Paleocene to early Eocene (~64-55 million years ago).

Around this time, all the modern mammalian orders were diversifying into a host of ecological niches vacated by the dinosaurs in the wake of the Cretaceous-Tertiary (K-T) Boundary mass extinction event 64.8 million years ago.

The plesiadapiforms are now regarded as proto-primates. Some resembled lemurs.

Pettigrew now invites us to join the dots: the paramomyids took to the air and evolved into colugos, which in turn, begat megabats. Meanwhile, their sister group, the plesiadapoids, evolved into our own ancestors, the primates.

And Pettigrew's scheme has recently found strong support, from a surprising quarter.

In the November 2 issue of Science, US and German molecular geneticists, led by Dr William Murphy of Texas A & M University, reported the results of their comparative DNA study of the three mammalian orders in the super-order Euarchonta: Primates (lemurs, marmosets, monkeys, and apes, including Homo), Dermoptera (colugos) and Scandentia (tree shrews).

They concluded that colugos are the long-elusive sister order to the primates and placed the divergence of primates and Dermoptera around 24 million years before dinosaur extinction - at 88.8 million years ago. The Scandentia were previous candidates as the sister group to primates, but they came much later - in the early Paleocene, around 63 million years ago.

But the latest analysis did not include fruit bats. The authors had no reason to include them in the quest for the sister group of primates - they were just bats.

What made the new analysis different was that it compared patterns of indels - INsertions and DELetions in the introns that divide genes into protein-encoding exons.

Where a point mutation simply swaps one base for another, indels physically change the length of a DNA sequence, by inserting two or more DNA bases at a specific point in the intron, or deleting two or more bases.

Pettigrew says that, with a point mutation, there is no telling in which of the two species the mutation occurred, without referring to other, related species. Over tens of millions of years, informative mutations become increasingly indistinct amid accumulating background "noise".

The US-German analysis did not employ DNA hybridisation - a technique developed in the late 1980s, where DNA from each of the species being compared is heated to the boiling point of water, separating the tightly bound strands of the double helix.

As the DNA mix cools, complementary sequences from the two species align and bind to each other; mismatched sequences remain separated. As they are reheated, the hybrid DNA separates at a temperature proportional to the time since the species diverged from a common ancestor. The lower the temperature, the longer the divergence time.

Indels are much rarer events. The genetic code is "read" in triplets, so any error that adds or deletes one or two bases creates a frameshift mutation that changes every amino acid downstream of that site, creating a nonsense protein. Non-lethal indels slowly accumulate, forming distinctive patterns across the genome. The more closely related the species, the more similar the patterns.