Megabat divergence

While the isochore problem does not invalidate DNA hybridisation as a tool for phylogeny, it may have caused molecular geneticists to underestimate divergence times within groups like birds, bats and insects.

Unless there is reliable fossil evidence, molecular geneticists rely on the youngest DNA-based estimate of when species diverged.

"When you're building evolutionary trees, if you begin with the assumption of a short distance between two species, it distorts all your other branch lengths," Pettigrew says. "Even a five per cent underestimate of the distance between two taxa distorts the tree, and in the case the megabat-microbat divergence, we're probably looking a 50 per cent underestimate.

"So I discarded the shortest distance, and relied on independent data from other researchers, including morphological comparisons."

Pettigrew decided to represent the relationships between four taxa as a three-dimensional tetrad, placing megabat, microbat and colugo at three vertices, and placing the tarsier (the famously big-eyed nocturnal primates) at the fourth vertex, representing the primate out-group.

Assuming the short megabat-microbat distance to be an artefact, he discarded it, and used the remaining five pair-wise distances to make a four-way comparison. "The results were very consistent," he says. "They all showed an underestimate of the megabat-microbat distance."

Tellingly, that underestimate was greatest for rhinolophid microbats, which have the second highest A-T content of any mammal, after megabats. The results support a close relationship between colugos and fruit bats, with humans next, and microbats being least related to the other three.

Pettigrew believes he is close to confirming that colugos and fruit bats are related, which would make them a sister group to the primates, by descent from a shared, 88 million-year plesiadapiform ancestor.

That would leave microbats out on a limb. When did they evolve, and from what ancestral glider? The fossil record offers no obvious candidate. A jaw, earbones and wingbones of one of the world's earliest fossil microbats, from Eocene (55mya) sedimentary rocks at Murgon in Queensland, are indistinguishable from today's species.

The fossilised egg of a noctuid moth from Martha's Vineyard, in Massachusetts, offers circumstantial evidence that echolocating bats were around 75 million years ago. (Noctuid moths have a unique 'ear' on the thorax that detects bat sonar, triggering a reflex that causes them to fold their wings and crash dive out of the bat's path.)

Mantises, which evolved in the early Cretaceous, (~125mya) also have a thoracic ear that triggers evasive action on detecting bat sonar, suggesting bats were among the earliest placental mammals.

The discovery of a primitive gliding mammal in inner Mongolia's Daguo fossil beds in 2005 extended the record for mammalian aerialists at least to the Jurassic-Triassic boundary, 140 million years ago. Not closely related to any modern or extinct mammal group, Volacotherium antiquum had teeth highly specialised for an insect diet - an unusual trait for a glider, says Pettigrew.

"Nobody is suggesting it is a microbat precursor, but it adds to the circumstantial evidence that bats could have evolved very early," he says.