On the cusp of the 30th Olympiad, as the world gawps at the apex of human movement, a team of Canadian scientists has published breakthrough research on the first creatures to move at all: prehistoric South American slugs.
Researchers from the University of Alberta have unearthed the oldest evidence yet of animals capable of self-propulsion. Earlier life forms, such as sponges, had to stay put.
As well, the 585-million-year-old slugs could be the first bilaterians – creatures with a front, back and sides. And they are the immediate ancestors of all locomoting animals, humans included.
The researchers’ findings, published in Science magazine’s June issue, pushes the oldest evidence of bilaterians back by 30 million years. (Previously, the earliest moving creatures were believed to be 555-million-year-old mollusk-like animals, found in shallow-water deposits in Russia.)
According to Ernesto Pecoits, a post-doctoral fellow at the university’s Department of Earth and Atmospheric Sciences and one of the study’s authors, the traces left by the tiny “slug-like animals” present evidence of “our most primitive and direct ancestor.”
The transition from sponges to slugs may not seem terribly impressive. But from an evolutionary perspective, the ability to move is “a huge step,” Prof. Pecoits says. Mobility means you can travel to hunt for food; you can burrow, build or escape.
The creatures in question were about as basic as animals get. Seven millimetres long and about two millimetres in diameter, they lived in northeastern Uruguay at the bottom of what was once an ocean. Contracting and relaxing muscles along their bodies let them glide, much like earthworms and your digestive tract. They also boasted primitive “feet” – tiny appendages they could extend into the silt to help move forward. Their footprints are preserved in tiny irregular holes poking out of fossilized burrows.
The slugs tunnelled through silt beneath a mat of algae on the ocean floor, popping up every so often for oxygen. Trails show they would poke freely into other slugs’ burrows, having yet to develop phobotaxis – an instinct to stay out of each other’s way that still eludes many Canadian urbanites.
Not unlike their homo sapiens descendants, the slugs got moving because they were hungry. They nibbled at bits of organic matter caught in the algal mat above them.
For their tiny pathways to have been set in stone, water near the ancient ocean’s floor would have to have been perfectly still or the burrows would disappeared as slabs of sediment settled like layers in a cake.
Fast forward several hundred million years. Prof. Pecoits and his fellow researchers were doing geological mapping, studying cross-sections of rock in Uruguay’s Tacuari formation, when they came across stone with clear traces of some squirmy creature’s path. But they had no idea how old the fossils were until they extracted zircon, a mineral that allows the rocks to be dated. The results were big news.
The challenge now is to connect the dots between these slugs and their spongey, homebody ancestors. As far as we know, the animal kingdom was dominated by immobile animals 635 million years ago. So what happened?
"This discovery will prompt new questions not only about the timing of animal evolution, but also the environmental conditions under which they evolved,” Prof. Pecoits said. “I think the community is going to be pretty excited.”