Researchers Discover That Our Ancient Ancestors Were More Complex Than Previously Thought

DNA Genetics Evolution Concept Art

The study also provides increased, but not conclusive, support for the existence of Xenambulacraria.

A new study by researchers at the University of Nottingham has shed light on the complexity of our ancient ancestors, solving an important piece of the animal evolution puzzle.

A new study by researchers at the University of Nottingham has revealed that our ancient ancestors were more complex than originally thought, solving an important piece of the animal evolution puzzle.

In the distant past, animals underwent a significant evolution by developing bilateral symmetry and two gut openings. This allowed them to move faster through the early seas, find food and extract nutrients more efficiently, and protect themselves from predators. The success of this trait can be seen in the diverse range of animals that still possess bilateral symmetry and two gut openings today, including humans, starfish, sea cucumbers, elephants, crickets, and snails. Additionally, a group of simple marine worms called Xenacoelomorphs also exhibit this trait, despite lacking the complex features of other animals.

For years, scientists have debated who is more closely related to who in this diverse collection of bilaterally symmetrical animals. Some experts argue that Xenacoelomorphs marks the first group to branch in that major jump in innovation from animals with circular body plans (e.g. jellyfish and corals) to bilateral symmetry. If this was the case, then the first bilaterian itself was also a very simple animal. Others argued for different placements of Xenacoelomorphs on the family tree.

However, a research team, led by Dr. Mary O’Connell at the University of Nottingham has found that Xenacoelomorphs branch much later in time. They are not the earliest branch on the bilaterian family tree and their closest relatives are far more complex animals, like starfish. This means that Xenacoelomorphs have lost many of the complex features of their closest relatives, challenging the idea that evolution leads to ever more complex and intricate forms. Instead, the new study shows that the loss of features is an important factor in driving evolution.

Dr. Mary O’Connell, Associate Professor in Life Sciences at the University of Nottingham says: “There are many fundamental questions about the evolution of animals that need to be answered. Many parts of this family tree are not known or not resolved. But what an exciting time to be an evolutionary biologist with the availability of exquisite genome data from the beautiful diversity of species we currently have on our planet, allowing us to unlock secrets of our most distant past.”

The study was recently published in the journal Current Biology. It details the application of a special phylogenetic technique to help in extracting signal from noise over deep time, showing increased support for Xenacoelomorphs being sister to ambulacraria (e.g. starfish) rather than being the deepest diverging of the bilateria.

The research team at the University of Nottingham will now explore other challenging family trees and other connections between genome changes and phenotypic diversity.

Reference: “Filtering artifactual signal increases support for Xenacoelomorpha and Ambulacraria sister relationship in the animal tree of life” by Peter O. Mulhair, Charley G.P. McCarthy, Karen Siu-Ting, Christopher J. Creevey and Mary J. O’Connell, 9 November 2022, Current Biology.
DOI: 10.1016/j.cub.2022.10.036

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