They are probably one of the oldest, albeit fleeting, remnants of evolution seen in humans yet, biologists say, in the journal Development.
They date them as 250 million years old – a relic from when reptiles transitioned to mammals.
It is unclear why the human body makes and then deletes them before birth.
The biologists say the developmental step may be what makes thumbs dextrous. Thumbs, unlike other digits, retain an extra muscle.
Rarely, some children and adults have been found to have a few of the extra finger and hand muscles but never all of the dorsometacarpales the biologists saw when they looked at 3D scans of embryos and foetuses at seven to 13 weeks’ gestation.
When they do persist, they are sometimes linked with limb deformities. And the biologists say their findings, in 15 developing babies, might help shed light on these types of birth defects.
Lead author Dr Rui Diogo, from the Howard University, in the US, said: “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.”
Useless body parts?
He said the structures were more striking than other evolutionary remnants humans retained, such as the apparently redundant appendix, wisdom teeth and coccyx.
“These muscles were lost 250 million years ago,” Dr Diogo said.
“No adult mammal, no rat, no dog has those muscles. It’s impressive. It was really a long time ago.
“It used to be that we had more understanding of the early development of fishes, frogs, chicken and mice than in our own species but these new techniques allow us to see human development in much greater detail.”
Dr Sergio Almécija, an anthropologist who studies ape and human evolution, at the American Museum of Natural History, said the findings provided a deeper appreciation of human development but raised many questions.
“The novelty of this study is that it allows us to visualise – with precision – when exactly during our development some structures appear and/or disappear,” he said.
“The important question for me now is, ‘What else are we missing? What will we find when all the human body is inspected at this detail during its development?
“‘What is causing certain structure to disappear and then to appear again? We can now see how it happens but what about the why?”
The biologists are planning more work looking at other parts of the human body in detail.
They have already studied the feet and know extra muscles develop and disappear there too while babies grow in the womb.
Monkeys and apes still have these muscles and use them to climb and manipulate objects with their feet.
Dr Diogo said: “Some of the things we are losing, it’s not that we are getting better humans and more progress. No. We are really losing things that will make super-humans.
“Super-humans would be keeping those muscles because you would be able to move all your digits, including your feet, as thumbs.
“We lost them because we do not need them.”