The basic design of the animal jaw has remained largely unchanged since it took shape in the depths of the seas some 400 million years ago, according to a study.

After a relatively brief period when a dizzying variety of jaw-like structures proliferated among backboned animals, the hinged mouth became the enduring model among vertebrates, researchers reported.

More than 99 per cent of vertebrates today, including humans, have jaws which share some variation of that rudimentary architecture.

But in the Devonian period, 420 million years ago, all of earth’s seas, lakes and rivers were dominated by toothless, armour-plated fish without jaws, which fed by sucking in algae and other small nutrients.

The study, published in Nature, challenges key assumptions about evolution during a time when our distant forebears were just emerging from the seas onto land. It had long been assumed that the rise of sea creatures with hinged mouths led to the rapid decline of the jawless fish which had populated earth’s early marine environment up to then.

Jawed fish – including the earliest sharks – would have been better hunters and scavengers, pushing the fixed-mouth species towards extinction, scientists reasoned.

But the new findings question that line of thinking.

“The variety of feeding mechan-isms in early jawed animals appears to have had little or no effect on the diversity of jawless fish,” said Philip Anderson, a professor at Britain’s University of Bristol and the study’s lead author.

For one thing, the two groups co-existed comfortably for at least 30 million years.

And when jawless fish finally did fade away there was no discernible evolutionary impact on their jawed cousins, suggesting that the two were not in direct competition.