Yale researchers have discovered what appears to be evidence of worm-like animals in rocks that are more than 1 billion years old -- about twice as old as any other evidence for multicellular life yet discovered. These findings, published in the Oct. 2 issue of the journal Science, add a new perspective to the origin of multicellular animals, typically thought to have begun with a sudden explosion during the early Cambrian period, about 540 million years ago.
Researchers from the University of Tübingen and Yale, as well as Jadavpur University in Calcutta, have found tunnels that may be burrows left behind as ancient worm-like animals wriggled through sand beds underneath a shallow sea covering what is now Central India. These structures, known as "trace fossils," were preserved when the beds solidified into rock 1.1 billion years ago. Before this discovery, the oldest known fossil evidence of multicellular animals was 580 million years old.
The Cambrian is often thought to have been the "big bang" of animal evolution, a time when a wide variety of organisms originated and left their mark on the fossil record. An important argument for this model of evolution has been that no multicellular organisms have been found in rocks older than the Cambrian. However, some molecular studies have suggested that soft-bodied animals arose well before the Cambrian, perhaps as much as 1 billion years ago.
The new findings, reported by Yale geology and geophysics professor Adolf Seilacher and his colleagues, add to the body of evidence suggesting that the diversification of animal designs experienced a "slow burn" before the Cambrian explosion.
The trace fossils are preserved in the Chorhat Sandstone, which contains sand beds that built up during storms. The tops of many sand beds were covered with a microbial mat that blanketed the floor periodically and protected the sand below from any disturbances above.
The ancient worm-like animals may have migrated through the sand just below the mats, using them as a source for food and oxygen since the water within the sand layers was probably "reduced," or poor in oxygen.
An important and often controversial consideration for researchers analyzing trace fossils is that physical processes can create patterns in sedimentary rocks that look very similar to tracks left behind by animals. According to Seilacher and his colleagues, the Chorhat findings are best explained as the products of burrowing animals.
Seilacher cites, for example, the fact that diameters vary from tunnel to tunnel but remain constant along each individual tunnel. The tunnels also do not resemble the structures commonly caused by physical processes, but are similar to younger trace fossils known to be produced by triploblastic animals (animals that developed from an embryo and that contain three outer membranes, as do worms).
The authors note in their paper that the existence of worm-like animals so much earlier than the Cambrian period would suggest that "animal body plans changed very little before the explosive emergence of new designs in the Precambrian/Cambrian transition and the onset of an arms-race style of Darwinian evolution."