The Evolution of Sperm: Tracing Its Origins Before Multicellular Animals

The evolutionary roots of sperm can be traced to the unicellular forerunners of all existing animals.

Nearly all animals go through a unicellular phase in their life cycle, which involves two forms of sex cells, or gametes. Eggs are sizeable cells that hold genetic information and the nutrients necessary for early development, while sperm transport genetic material from one organism to another to fertilize eggs and create new life.

“Sperms play a crucial role in the process that allows life to be transmitted from generation to generation,” states Arthur Matt from Cambridge University. “It carries the legacy of over 700 million years of evolutionary history and is likely linked to the origins of animals themselves. Our aim was to explore this extensive evolutionary narrative to understand the origins of sperm.”

Matt and his team utilized an open science dataset containing information about sperm proteins from 32 animal species, including humans. They combined this data with the genomes of 62 organisms, including various related single-cell groups, to track the evolution of sperm across different animal lineages.

The research revealed a “sperm toolkit” comprising about 300 gene families that make up the last universal common sperm core genome.

“We have now identified numerous significant advancements in sperm mechanisms occurring long before multicellular animals emerged, even before the sperm themselves,” explains Matt.

This indicates that the sperm mechanics, represented by a “flagellum that propels a single cell,” were already evolving prior to the development of multicellular organisms.

Thus, our ancient progenitors were once all single-celled oceanic swimmers, and the sperm toolkit was present in our earliest swimming unicellular predecessors long before the advent of animals.

“Animals evolved multicellularity and cellular differentiation, but they did not create sperm from nothing. They repurposed the body structure of their swimming forebears as the foundation for sperm,” states Matt. “In essence, sperm are not a novel creation of multicellular organisms but are constructed upon the designs of a single-celled organism repurposed for reproduction.”

The study also indicated that the significant technological developments leading to the vast variety of current sperm primarily affected the cell heads, while the tails have remained largely constant since their common ancestor.

According to the research team members, fertilization can occur in various manners, with some sperm reaching the egg within the body, while others swim in open waters, notes Adria Leboeuf, also from the University of Cambridge. “Finding eggs in these different settings presents unique challenges and requires specialized machinery,” she explains. “However, the tail remains well-preserved since it must be capable of swimming in all environments.”

“This illustrates how evolution can modify existing structures instead of creating mechanisms from scratch,” says Jenny Graves, from La Trobe University in Melbourne, Australia.