If you have ever described someone as resembling a toad, you're almost right - and now scientists know why.
It's an established fact that animals with a backbone briefly look alike during the phylotypic stage of embryo development before diverging to become different species.
But now biologists have discovered how the same genes are turned on early in embryonic development to make the animals look so similar.
Scientists at the University of Western Australia worked with researchers at the Spanish National Research Council and Radboud University in the Netherlands, examining mice, fish and toads.
Lead author Ozren Bogdanovic, from the ARC Centre of Excellence in Plant Energy Biology, says the study shows how chemical signposts change in the DNA during the phylotypic stage.
He says they alter in a wave and activate the same developmental pathways in each animal.
But the time taken to reach the phylotypic stage varies between species.
"In fish and toads it would be at one to two days after fertilisation, and at nine-and-a-half days in mice, while humans go through the phylotypic stage about four weeks after conception," Dr Bogdanovic said.
"It's likely that we also have a similar type of epigenetic control in our development during that period.
"If you were to put a human embryo next to a fish, a toad and a mouse at that stage, the human embryo would look very much like the others."
Professor Ryan Lister said it was thought vertebrates showed such similarity during the developmental period because that was when the fundamental structure of the body was set up.
"It's fascinating to see these similarities right down to the molecular level, and we can do so only because of recent technological advances in DNA sequencing that give us the power to dig much deeper into biological systems," he said.
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