Humans may owe their place on the planet to a mass extinction of fish 360 million years ago, it was claimed.
The cataclysmic event reset the evolutionary starting point for all vertebrates living today, said US scientists.
If it had not occurred, humans and their ancestors may not have evolved - or could have evolved very differently.
Key features shared by all modern mammals, birds and reptiles - such as five-digit limbs - originated when life re-emerged after the mass extinction, the experts believe.
"Everything was hit, the extinction was global," said researcher Lauren Sallan from the University of Chicago. "It reset vertebrate diversity in every single environment, both freshwater and marine, and created a completely different world."
The Devonian Period, which stretched from 416 to 359 million years ago, is also known as the Age of Fishes.
A broad array of species filled the oceans, rivers and lakes, but most were unlike any alive today.
Armoured placoderms, such as monstrous 30-foot carnivore Dunkeosteus, and lobe-finned fishes similar to modern lungfish dominated the waters. Ray-finned fishes, sharks and four-limbed tetrapods were in the minority.
But the picture changed abruptly with the traumatic Hangenberg extinction.
"There's some sort of pinch at the end of the Devonian," said Professor Michael Coates, from the University of Chicago. "It's as if the roles persist, but the players change: the cast is transformed dramatically.
"Something happened that almost wiped the slate clean and, of the few stragglers that made it through, a handful then re-radiate spectacularly."
New fossil finds and analytical techniques brought to light the full impact of the Hangenberg event, said the scientists. The research is published in the journal Proceedings of the National Academy of Sciences.
What happened to trigger the mass extinction remains an unsolved mystery.
Other scientists found evidence of substantial glacier formation at the end of the Devonian which would have dramatically affected sea levels.
The first appearance of forest-like environments may also have produced atmospheric changes with catastrophic consequences for life.
"It is a pivotal episode that shaped modern vertebrate biodiversity," said Prof Coates. "We are only now beginning to place that important event in the history of life and the history of the planet, which we weren't able to do before."Reuse content