Platelets are tiny blood cells that help our bodies form clots to stop bleeding. They are found in all mammals but no other group of animals. According to new research from Yale University and University College London, these cells first evolved approximately 300 million years ago (Carboniferous period) in an egg-laying animal similar to the modern duck-billed platypus; this event was a prerequisite for the origin of placental development in mammals, including human beings.
An artist’s reconstruction of an ancient duck-billed platypus. Image credit: Peter Schouten.
Platelet cells were critical in the evolution of eutherian mammals, to which humans belong, and which are distinguished by a deep invasive placenta (hemochorial placentation), by where maternal blood comes in direct contact with the fetus.
These cells enabled hemochorial placentation, helping the mother prevent hemorrhaging at birth.
“The unique presence of platelets in mammals explains why deeply invasive placentation is limited to mammals, even though live birth is found in many other animal lineages, but not invasive placentation,” said Yale Professor Günter Wagner.
In a paper published in the journal Biology Letters, Professor Wagner and University College London’s Professor John Martin show that an ancient egg-laying animal similar to a duck-billed platypus started creating platelets, possibly by chance.
These were passed on when this animal group diverged around 300 million years ago into monotremes (the first mammal group), of which the existing duck-billed platypus and echidna are living descendants, marsupials (also mammals) and eutherian mammals, which include modern humans.
“We have shown with convincing evidence that platelets occurred 300 million years ago even before monotremes arose,” Professor Martin said.
“This unique feature subsequently allowed the placenta to develop, which led to the eutherian mammals and therefore human beings.”
“During birth, safe disconnection of the placenta from the uterus is essential for the survival of the mother and child, so without platelets, neither would have survived and the evolutionary step to eutherian mammals, including human beings, would never have happened.”
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John F. Martin & Günter P. Wagner. 2019. The origin of platelets enabled the evolution of eutherian placentation. Biology Letters 15 (7); doi: 10.1098/rsbl.2019.0374
