There's so much that we don't know about the evolutionary world, but a new study published in the journal Development has brought us one step closer - and it all starts in the womb.
Babies in the womb have extra lizard-like muscles in their hands that most will lose before they are born. Dating back 250 million years, they are probably one of the oldest remnants of evolution seen in humans yet, biologists said.
Medical scans revealed the extra lizard-like muscles - a relic from when reptiles transitioned to mammals. And yet, it is unclear why the human body makes and then deletes them before birth.
But biologists said the developmental step may be what makes thumbs dextrous.
Lead author Dr Rui Diogo, from the Howard University in the US, told BBC.com: "We have a lot of muscles going to the thumb, very precise thumb movements, but we lost a lot of muscles that are going to the other digits.
"In our evolution, we do not need them so much.
"Why are they there? Probably, we cannot just say in evolution, 'Look, I will delete from scratch, from day zero, the muscle going to digits two, three, four, five and I will just keep the one going to the thumb.'
"Probably it is not so easy. Probably you have to form this layer of this muscle and then it disappears on the other digits but persists on the thumbs."
He added that the structures were more striking than other evolutionary remnants humans retained.