20 February 2015 Last updated at 00:02
By Jonathan WebbScience reporter, BBC News
Whales and other modern sea animals tend to be much larger than Cambrian sea creatures
The animals in the ocean have been getting bigger, on average, since the Cambrian period - and not by chance.
That is the finding of a huge new survey of marine life past and present, published in the journal Science.
It describes a pattern of increasing body size that cannot be explained by random "drift", but suggests bigger animals generally fare better at sea.
In the past 542 million years, the average size of a marine animal has gone up by a factor of 150.
It appears that the explosion of different life forms near the start of that time window eventually skewed decisively towards bulkier animals.
Measured by volume, today's tiniest sea critter is less than 10 times smaller than its Cambrian counterpart; both are minuscule, sub-millimetre crustaceans. But at the other end of the scale, the mighty blue whale is more than 100,000 times the size of the largest animal the Cambrian could offer: a trilobite less than half a metre long.
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