Why hasn't evolution changed some animals?

Animals are younger than expected

Chemically modified fossil algae molecules have falsified the evidence of the first sponge-like animals

A longstanding controversy about the origin of complex life on earth has now apparently been resolved. A team involving researchers from the Max Planck Institute for Biogeochemistry has found that fossil fat molecules isolated from 635 million year old rocks are not the earliest evidence of animals. The fossil molecules, which are similar to the steroid molecules of sponge-like animals, are formed by geological processes from precursor molecules of common algae, which are geologically much older than animal life forms. The evolution of animals began later than some findings previously suggested.

As early as 2009, researchers found fossil lipid molecules, ancient steroids, in rocks more than 635 million years old, which they believed were remnants of sea sponges. Since sponges are among the oldest and simplest representatives of the animal world, these finds were interpreted as the earliest traces of animal life. Larger sponge fossils that would match the steroid molecules were never discovered. "One of the great mysteries of early animal evolution was that there were no unambiguous fossils from this period, but chemical remains of spongy animals seemed to be abundant," says Benjamin Nettersheim, who works at the Max Planck Institute for Biogeochemistry in Jena and on the Marum - Research Center for Marine Environmental Sciences at the University of Bremen.

Geological processes turn algae molecules into animal steroids

The riddle is now solved by two independent experimental studies. "We were able to show that molecules from normal algae can change chemically under the influence of geological processes in such a way that they become the said steroid molecules," says Lennart van Maldegem from the same working group. "These supposedly fossil molecules cannot be separated from those from sponge-like animals that were found in fossils 100 million years younger. "

In a second study, a group from the Australian National University and the California Institute of Technology confirmed the finding. “It's true that sponges are the only living organisms that can produce these steroids. But chemical conversions can turn simple steroids from plant algae into supposedly animal steroids under special geological conditions, "says Ilya Bobrovskiy, who studies the origin of animal life at the Australian National University.

A clear time frame for the evolution of animals

Both research teams provided evidence of the evolutionary deception in the chemistry laboratory: They used steroids extracted from algae or synthetic substances as starting materials. From these they produced the animal steroids using pyrolysis techniques and thus simulated the geological processes.

As early as 2019, a team led by Benjamin Nettersheim and Christian Hallmann showed that the steroid finds that were previously attributed to animals could also come from protists. These unicellular organisms, which are informally counted among the protozoa or ancient animals, are widespread in modern oceans. In modern ancient animals, however, only small amounts of the corresponding steroids have been detected. The new results show that the traces in the ancient rock are unlikely to have come from animals or protozoa.

Christian Hallmann, professor at the University of Potsdam and head of one of the two current studies, underlines the importance of the new findings for our understanding of evolution: “Understanding the rise of animals is so incredibly important because it is at the root of our very existence stands. Before we can investigate which factors drove the development into complex organisms, we must first clearly define the time frame for evolution. "

Up until the Cambrian there was too little oxygen for animal life

The appearance of the first animal organisms has been the subject of intense and controversial discussions in the last few decades. "Our results bring the oldest evidence for animals almost 100 million years closer to the present," says Benjamin Nettersheim, "that is to say about 560 million years before our time." According to this, creatures of the Dickinsonia genus are the first animals, as a team in which Benjamin Nettersheim and Christian Hallmann were also involved, in addition to researchers from the Australian National University, discovered in 2018.

The later starting point for animal life, which the current studies now underpin, also fits in much better with what is known about the ecological demands of animals and the environmental conditions at the time: "We know that animals need habitats with a relatively high oxygen content in order to be able to to achieve high ecological relevance, ”explains Christian Hallmann. "Up until the beginning of the Cambrian, about 540 million years ago, there was relatively little oxygen in the oceans." However, this ultimately led to the Cambrian explosion, in which countless new species developed rapidly during the Cambrian, including the forerunners of all animal phyla known today , remains controversial - this is one of the questions Hallmann and his team will be tackling now.

EF / PH