The worm that gave the world symmetry The prototype of all 'two-sided' animals on earth has surfaced...
Written By Tim Radford Edited By Isabel Cutter
An Anglo-Spanish team of scientists has discovered the oldest living ancestor of the bilateral world. Any creature from poker players to partridges, from aardvarks to aphids with right and left hand sides, is cousin to a tiny flatworm that first lived on Earth 530 million years ago.
The acoel lives in sand or mud below water, the world over. It dines on microbes, and has spent most of the past 500 million years keeping out of sight.
At first, the scientists of the University of Barcelona and London's Natural History Museum, did not know how to classify it as it seemed to have no defining characteristics.
In the US journal, Science, they report today that it was the first two-sided creature to make its bow in a world occupied until then largely by blobs, such as the ancestors of sponges and jellyfish.
It was the missing link in the ancestry of creatures which have fronts and backs. 'They have always been simple and they have been on the planet a very long time,' said Tim Littlewood, of the Natural History Museum. 'If we are going to understand how we all came about we need to look at these as well.'
Life began on the planet more than 3 billion years ago but for the first 2 billion years was occupied only by single-celled bacteria.
About 600 million years ago came the first multi-celled animals. They had soft bodies and left behind few traces; by the arrival of the first animals with shells and bones, evolution was well advanced. The origin of most living creatures is a mystery, but scientists using DNA analysis been able to order groups of creatures.
They were surprised to find that acoels were the ancestors of everything else with two sides.
'They have very little going for them,' said Dr Littlewood, adding that biologists had little to get their teeth into when it came to classifying the animals. 'But if you were to say, 'I want an archetypal body plan to start with, what am I going to build on?' this would be a good one to start with.'
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