Ribozyme crevices and catalysis.

scaled down to nearer 670 million years5, bringing the figures more into line with the fossil record. A 'late arrival' model would also imply that the evolution of the main animal groups took place both late in Earth's history, and rapidly-perhaps in response to the evolution ofHox genes6 or to the lifting of some external constraint such as insufficient atmospheric oxygen2. So should these supposedly ancient markings be accepted as new evidence for the 'deep time' model? When a respected researcher such as Seilacher, who has a good track record in the field of trace fossils, makes such a claim as this, it has to be taken seriously. But extraordinary claims, of course, require extraordinary evidence. The first claim' is that these are the traces of 'triploblastic' animals. This means that the trace-makers had a gut and a fluid-filled coelom.Such an organization is found only in complex animals from worms to chor-dates. Seilacher and co-authors' argument is that, at 5 mm, the burrow diameters are too large to have been made by single-celled protists; but unicellular protists, including early Cambrian forms, can be that large7 and can make burrows. Moreover, at 5 mm diameter, how could such organisms have remained hidden from the fossil record for over 500 million years? Finally, how come the tunnels are branched? Branching implies a behavioural sophistication which is thought to have appeared about 560 million years ago'. These seeming paradoxes may yet be explained by a fresh look at the age of the lower Vindhyan rocks (Fig. 1). Until now, they have broadly been taken to be 1,100 million years old on the basis of potassium -argon and fission-track dates. But these dates were mostly obtained in the 1960s, and no further particulars are given by the authors'. Unfortunately, the data they cite must now be regarded as a very doubtful guide to the age ofthevindhyan for which modern studies of geochronology are urgently required. In fact, we may be in for a big surprise regarding the age of the Vindhyan super-group, including, of course, the Chorhat sandstone. Acid digestion of the immedi-a tely overlying Roh tasgar limestones, undertaken in India by R. J. Azmi, has yieldEd a rich, well-preserved and typical early Cambrian skeletal fauna, including certain types of brachiopod". This astounding iiscovery means that supposed trace-fossil markings from the Chorhat sandstone may)e little more than 540 million years old, :lose to the …