Your oldest ancestor was a sack-like creature with no anus

The bag-like invertebrates are less than 1.2 mm long with pleated, circular mouths used to feed and excrete waste

30 Jan 2017
Simon Conway

Analysis of tiny fossilised creatures that lived during the early Cambrian period has revealed new clues about our most ancient ancestors.

The bag-like invertebrates, found in Shaanxi, China are less than 1.2 mm long with pleated, circular mouths and up to eight openings along their bizarre bodies. Classified as deuterostomes, a diverse group of animals of vertebrates (including us), starfish and acorn worms, it is believed that the creatures lived in water around 540 million years ago, making them the earliest known deuterostomes to date – and as a result, our oldest relatives.

The study, led by Simon Conway from the University of Cambridge and published in Nature, describes the small animals as having "bag-like bodies" with prominent mouths and four conical openings on either side.

Simon Conway Morris/University of Cambridge

These openings would not have been used for breathing, according to the researchers, but may have eventually evolved into gill slits seen in modern water-dwellers. Unusually for deuterostomes, though, the animals appear to have had no anus, so the openings may have been used to both feed, and excrete water and waste. This mouth was disproportionately large for the creature's size, too, meaning it could have eaten other creatures.

The study adds that the creature's body was symmetrical, a trait seen in its evolutionary descendants, including humans, and it was covered with a thin skin and muscles. It likely moved by "wiggling" and contracting the muscles along its body.

Simon Conway Morris/University of Cambridge

In a related paper, published in Nature Ecology & Evolution, Thomas Harvey and Nicholas Butterfield present a different set of tiny animal fossils from later in the Cambrian period — loriciferans from Canada.

Harvey, from the University of Leicester and the University of Cambridge's professor Butterfield found the new species while carrying out a survey of microfossils in mudstones from western Canada. To their surprise, the samples yielded minuscule loriciferans: a type of animal so small it has been considered "unfossilisable".

Simon Conway Morris/University of Cambridge

They date to the late Cambrian Period, half a billion years ago. This suggests that soon after the origin of animals, some groups were adopting specialised "meiobenthic" lifestyles, meaning they lived among grains of sediment on the seabed.

"I discovered the fossil loriciferans by accident while surveying other types of microfossil: this required many hours working at the microscope," said Harvey. "I kept finding mysterious fragments which looked like the back ends of loriciferans, but I told myself it was impossible.