Muscles thought discarded during the long descent that shaped humanity are in fact still present in embryos – providing a fascinating insight into the workings of evolution.
That’s the central finding arising from the sophisticated three-dimensional imaging of 36 first trimester embryos and foetuses, carried out by Rui Diogo and Natalia Siomava, from Howard University College of Medicine in the US, and Yorick Gitton from the Sorbonne in France.
Using recently developed staining and high-resolution imaging techniques, the trio were able to track the development of limb muscles across the first 13 weeks of development. They discovered that several muscles absent in modern humans but present in ancient hominin ancestors, other primates, and in some much more distantly related species of four-legged animals, still develop during the early weeks of gestation.
The muscles included those known as epitrochleoanconeus, dorsoepitrochlearis, contrahentes, and dorsometacarpales, all in the upper limb, and contrahentes, dorsometatarsales, and opponens digiti minimi in the lower limb.
Of these, the dorsometacarpales merged with other muscles by the end of the first trimester, while all the others simply disappeared.
Diogo and colleagues note that in some rare cases the muscles have been retained in adult humans, where they are regarded as congenital malformations – albeit, generally speaking, not very serious ones. The discovery that they form as a standard part of foetal development, they say, “reinforces the idea that such variations and anomalies can be related to delayed or arrested development”.
The imaging carried out by Diogo and colleagues represents the first major insight for decades into how tissues develop in embryos.
And although the existence of vestigial muscles was unexpected, it was not, in another sense, surprising. It has long been noted that physical structure in species today reflects in many cases the shedding of appendages rendered unhelpful by shifts in habitat.
Ostriches and emus, for instance, have stubby little wings, which are remnants of altogether more useful and bigger ones that were used by their distant flying ancestors. Deep beneath their blubber, whales and dolphins possess the bony remnants of the legs that their predecessors used to walk on land.
And at the base of every human spine lies a coccyx – a reminder that millions of years ago our ancestors had tails.
The research is published in the journal Development.