Irish and US researchers redraw tree of life and settle age-old scientific dispute


RESEARCHERS FROM Ireland and the US have redrawn the tree of life. It helps explain an explosion of new species that emerged 530 million years ago, but also manages to settle a long-standing scientific dispute in the process.

The “Cambrian explosion” was a time when evolution on Earth went into top gear. Single-celled organisms that dominated 580 million years ago gave way in no more than a geological heartbeat to a level of species complexity like we see today.

The explosion of new lifeforms was noted in Charles Darwin’s day in the mid-1800s. There was a wealth of fossils back to the Cambrian but then nothing, as if nature wasn’t working at all.

The dramatic appearance of so many novel Cambrian species over such a short time also seemed to fly in the face of Darwin’s new explanation for life on Earth, a process of gradual rather than cataclysmic evolution.

For this reason scientists have long doubted whether the Cambrian explosion was real at all, explained Dr Davide Pisani.

The NUI Maynooth-based lecturer in bioinformatics joined with scientists from Harvard, Yale, Dartmouth and the Smithsonian Institution to help settle the dispute. They also confirmed that Darwinian evolution did hold sway despite the explosion. Their work is published this morning in the journal Science.

The researchers decided to combine two ways of looking back through time. One involved looking at the fossil record and what it told them about when species arose.

The second involved using genetic markers in all sorts of animals living today as a way to look backwards using something called the “molecular clock”, Dr Pisani said.

Nature loves a good idea and once found it tends to leave it alone. This means it survives in generation after generation as a limb of the tree of life grows and branches.

But this allows researchers to peer backwards in time as well, looking for the places where two smaller limbs join to form one. Using this method, researchers know a common ancestor of mammals and birds lived 310 million years ago, Dr Pisani said.

When the fossil record and data from the molecular clock were matched up it became clear that things were actually happening in the millions of years before the Cambrian.

The first sponges were thought to have arisen about 700 million years ago on the basis of the molecular clock. Gradual evolution was taking place despite the lack of fossils in the record, he said. “Before the split between the jellyfish and vertebrates the [evolutionary] system was there. We had gradual evolution that allowed species to develop.

“What we showed is that it [the Cambrian explosion] is actually real but it is not in disagreement with the gradual development of evolution,” he said.