Have you ever wondered what your oldest ancestor looked like? Well, according to some scientists, it was a microscopic, bag-like sea creature that lived 540 million years ago and had a very large mouth, but no anus.
The peculiar creature is known as Saccorhytus, and it is a member of the deuterostome group of animals, which encompasses vertebrates, echinoderms, and hemichordates. Deuterostomes are distinguished by their embryonic development, which involves the formation of a mouth and anus that are independent of each other.
“We think that as an early deuterostome this may represent the primitive beginnings of a very diverse range of species, including ourselves. All deuterostomes had a common ancestor, and we think that is what we are looking at here,” said Prof Simon Conway Morris, from the University of Cambridge.
Saccorhytus fossils were found in Shaanxi Province, in central China, and pre-date all other known deuterostomes. The fossils were remarkably well preserved and studied using electron microscopy and CT scan.
“Saccorhytus now gives us remarkable insights into the very first stages of the evolution of a group that led to the fish, and ultimately, to us,” said Degan Shu, from Northwest University in Xi’An, Shaanxi Province.
Saccorhytus probably ate and excreted through the same opening. It also had small conical structures on its body that may have been the precursors of gills.
However, not everyone agrees that Saccorhytus is our earliest ancestor. A recent study published in Nature reinterpreted Saccorhytus as an early ecdysozoan and not the earliest deuterostome, based on new material and phylogenetic analysis. Ecdysozoans are a group of animals that shed their exoskeletons during growth, such as arthropods and nematodes.
“Saccorhytus is an important fossil that seems to represent the oldest known deuterostome. Together with vetulicolians, it might be among the earliest deuterostomes, in which case it could shed light on the order in which characteristics were acquired in the period leading up to the divergence of the living deuterostome groups,” said Douglas H. Erwin, from Nature.
The debate over Saccorhytus shows how challenging it is to reconstruct the early history of life on Earth. But it also shows how fascinating it is to discover new clues about our origins and our connections with other living beings.
“The implication is that the human genome arose in Africa. Everyone is African, and yet not from any one part of Africa,” said Rick Potts, director of the Smithsonian’s Human Origins Program.
– Our Oldest Ancestor Was A Microscopic Big-Mouthed Sea Sack, Forbes, January 31, 2017
– Saccorhytus is an early ecdysozoan and not the earliest deuterostome, Nature, December 22, 2021
– Is this ancient, bag-like sea creature our earliest ancestor?, CNN, December 21, 2021
– Tiny fossils in the animal family tree, Nature, December 19, 2021