DNA study reveals more of the tree of life

06 March 2008

A genetic study reported in the journal Nature, answers some old debates about animal evolution, including what could be the new lowest animal form and what the closest relations of millipedes and centipedes are.

Charles Darwin introduced the idea of a tree of life in his publication The  Origin of Species, to try and explain the evolutionary relationships between all the different species of animals on Earth. Nearly 150 years after its publication, relationships between many animal groups remain unclear.

Now, the most comprehensive animal genetic research project to date has been carried out by a team of scientists including Greg Edgecombe at the Natural History Museum and Casey Dunn at Brown University. Their research has settled some of these long-standing scientific debates.

Understanding the tree of life

There are millions of species on the planet and we may have to wait a very long time before each one of them has their genetic information recorded.

Velvet worm © Thomas Stromberg/ Wikipedia

Velvet worm © Thomas Stromberg/ Wikipedia

So, to get a better overview and understanding of the whole tree of life, the team decided to focus on 29 poorly understood animals that are at the very end of widely-separated branches in this tree.

They collected genetic data from animals including comb jellies, moss animals, peanut worms, spoon worms, horsehair worms, centipedes, sea spiders, velvet worms and molluscs.

The team analysed these data with existing data from 48 other animals, such as humans and fruit flies, and they looked for the most common genes being activated, or expressed.

Comprehensive research

The new research has added 40 million previously unknown base pairs of DNA data from the 29 species.

Peanut worm

Peanut worm

As well as being the most comprehensive animal genetic research project to date, the new process was also more computationally intensive. The project demanded the power of more than 120 processors housed in computer clusters located in laboratories around the world.

Comb jellies lowest form of animal life

One of the surprising findings moves the sponge from the bottom spot of the animal tree of life, when multi-cellular life began to diversify hundreds of millions of years ago.

The team discovered that a common and extremely fragile group of animals called comb jellies diverged from other animals even before the lowly sponge. Interestingly, sponges have no tissues at all whereas the comb jellies have well-developed tissues and a nervous system.

'Coming up with these surprises, and simply trying to better understand the relationships between living things, made this project so fascinating,' says Dr Dunn, who led the study.

'This finding suggests that either comb jellies evolved their complexity independently from other animals, or that sponges have become greatly simplified through the course of evolution. If corroborated by other types of evidence, this would significantly change the way we think about the earliest multi-cellular animals.'

Millipedes and centipedes more like spiders

Another issue debated for years was how the jointed-legged animals in the arthropod group are related to each other, and especially whether millipedes and centipedes are more closely related to insects or to the group that includes spiders, scorpions and mites. The new research has found that spiders are closer relatives to millipedes and centipedes than insects.

Other close ties

The team also established some previously undetected animal relationships, such as the close ties between nemerteans (ribbon worms) and brachiopods (lamp shells) and a larger grouping of those phyla (as yet unnamed) with the annelids (segmented worms) .

'One of the really encouraging aspects of this study,' says Dr Edgecombe, 'is that we are able to find clear genetic evidence for several great groups of animals, like segmented worms and molluscs, that had long been recognised by scientists working with anatomy but hadn’t been well-supported by smaller-scale analyses using genetic data.'

Future research

As well as answering many long-debated issues, this research will enable evolutionary scientists to unveil more answers in the years to come.

'This approach looks like a very powerful tool for delivering a stable, well-supported evolutionary tree for the animals as a whole,' Dr Edgecombe concludes. 

The project has been supported by grants from the US National Science Foundation under its Assembling the Tree of Life initiative.