The swimming mechanism of sperm has ancient origins.
Christoph Burgstedt/Alamy
The evolutionary origins of sperm can be traced back to the single-celled ancestor of all living animals.
Almost all animals reproduce by going through a single-celled stage of the life cycle involving two types of sex cells, or gametes. Eggs are larger cells containing genetic material and resources for early development, while sperm transport genetic material from one body, find the egg and fuse with it to form a fertilized zygote.
“Sperm carries the mechanism that allows life to be passed on from one generation to the next,” says Arthur Matt at Cambridge University. “It retains traces of more than 700 million years of evolution and is likely related to the origins of the animals themselves. We wanted to trace this long evolutionary history to understand where sperm came from.”
Matt and his colleagues used open science datasets containing information about the proteins that make up the sperm of 32 animal species, including humans. They then combined this data with the genomes of 62 organisms, including some groups of single-celled animals, allowing them to trace the diversification of sperm across different animal lineages.
They found that a “sperm toolkit” of approximately 300 gene families constituted the core genome of the last universal common sperm.
“We could see that many sperm mechanisms had major innovations before metazoans existed, long before sperm themselves existed,” Matte says.
This suggests that the sperm mechanism, “a flagellum spinning around a single cell,” had already evolved before multicellular animal life arose, he says.
This means that our distant ancestors were once single cells swimming in the ocean, and the tool kit of sperm first formed in a swimming single-celled ancestor, long before the appearance of animals.
“As animals evolved multicellularity and cell specialization, they did not invent sperm from scratch; they reused the body structure of these swimming ancestors as the basis for sperm,” Matte says. “In other words, sperm is not a flashy new invention of multicellular life; it is built on the basis of a single-celled body repurposed for reproduction.”
The study also found that the innovations that led to the enormous diversity of modern sperm mainly changed the head of the cell, while the tail has changed little since the time of the common ancestor.
According to the team member, there are many different ways of fertilization: some sperm meet eggs inside the body, while others float in the open ocean. Adria LeBoeufand also at the University of Cambridge. “Finding eggs in different environments will be different and require different equipment,” she says. “But no matter where you are, you still have to swim, so the tail is quite preserved.”
“This is a great example of how evolution changes what exists, rather than inventing mechanisms from scratch,” says Jenny Graves at La Trobe University in Melbourne, Australia.
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