Sperm’s evolutionary origins go back before multicellular animals

The evolutionary origin of sperm can be traced back to a single-celled ancestor of all living animals.

Almost all animals reproduce by having a single-celled stage of their life cycle, involving two types of sex cells, or gametes. Eggs are larger cells containing genetic material and resources needed for early development, while sperm carry genetic material out of a body, locate an egg, and fuse with it to create a fertilized zygote.

“Sperm carries the machinery that allows life to pass from one generation to the next,” explains Arthur Matte of the University of Cambridge. “It preserves traces of more than 700 million years of evolution and is likely linked to the origin of animals themselves. We wanted to trace this long evolutionary history to understand where sperm come from.”

Matte and his colleagues used open science datasets containing information on 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 single-celled groups related to animals, allowing them to trace sperm diversification across animal lineages.

They discovered that a “sperm toolbox” of around 300 gene families made up the core genome of the last universal common sperm.

“We were able to see that a large part of the sperm machinery had undergone major innovations even before the existence of multicellular animals, well before the spermatozoa themselves,” explains Matte.

This suggests that the sperm machinery, “a flagellum growing around a single cell,” had already evolved before the emergence of multicellular animal life, he says.

This implies that our distant ancestors were once all single cells swimming in the ocean, and that the sperm toolkit first took shape in a single-celled, swimming ancestor, long before animals existed.

“As animals evolved multicellularity and cellular specialization, they didn’t invent sperm from scratch; they reused the body plan of these swimming ancestors as the basis for sperm,” says Matte. “In other words, sperm is not a flashy new invention of multicellular life; it is built on a single-celled body plan repurposed for reproduction.”

The study also found that the innovations that led to the immense diversity of modern sperm have primarily changed the head of the cell, while the tail has changed little since the common ancestor.

There are many different modes of fertilization, with some sperm meeting eggs inside a body and others swimming in the open ocean, says team member Adria LeBoeuf, also at the University of Cambridge. “Finding an egg in these different environments will be different and will require different machines,” she says. “But wherever you are, you’ll still have to swim, so the tail is pretty conserved.”

“This is a great example of how evolution works to reshape what exists rather than inventing mechanisms from scratch,” says Jenny Graves of La Trobe University in Melbourne, Australia.