'Larger population sizes make natural selection more efficient,' said IU-B evolutionary biologist Michael Lynch, who led the study.

'If we are correct, we have shown how to bring ancient genomic information together with the palaeontological record to learn more about the past,' said Lynch.

Lynch says the data he and his colleagues analysed suggest human genomes are still undergoing a contraction -- though humans shouldn't expect to see noticeable changes in their chromosomes for a few million years yet.

The study was published in the inaugural issue of the journal Genome Biology and Evolution.