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What Did the Earliest Animals Look Like?

Researchers use chromosome structure to identify the first lineage to branch off from the animal tree, ctenophores, followed by sponges and all other animals, including humans.

Low Res Hormiphora californensis Darrin Schultz MBARI 2048.JPG
Photo: California sea gooseberry, is a comb jelly, or ctenophore, Credit: Monterey Bay Aquarium Research Institute

Biologists have been searching for the earliest animals for more than a century, narrowing the possibilities down to two groups: sponges and comb jellies.

Researchers employ a novel method based on chromosome structure in a recent study just published in Nature to provide a conclusive response (Schultz et al., 2023).

The ctenophore lineage branched off before sponges, then diversifying all other animals, including the lineage leading to humans.

However, evolutionary scientists think that these groupings still have traits in common with the earliest animals and that research into this early animal tree of life branches will help us understand how animals came to be and developed into the variety of species we see today.

Understanding the relationships between animal lineages will enable scientists to comprehend the evolution of important aspects of animal biology, such as the digestive system, the muscles, and the nervous system.

The development of a new way to take one of the deepest glimpses possible into the origins of animal life has laid the basis for the scientific community to better understand how animal evolution has occurred.

Most familiar animals, such as worms, flies, mollusks, sea stars, and vertebrates, a gut running from mouth to anus, have a head with a centralized brain, muscles, and other shared features that had already evolved by the time of the famed “Cambrian Explosion” around 500 million years ago.

Other bona fide animals, such as jellyfish, sea anemones, sponges, and ctenophores, have simpler body plans but still share the hallmarks of animal life, notably the development of multicellular bodies from a fertilized egg.

The evolutionary connections between these many organisms have been the subject of debate, particularly the timing of when each lineage diverged from the animal tree of life’s central trunk.

DNA sequencing has allowed biologists to compare the sequences of genes in different species, thereby creating a family tree.

The most important details in this text are that sponges have traditionally been considered the earliest surviving branch of the animal tree and that comb jellies are also a candidate for the earliest animal lineage.

To learn whether sponges or ctenophores were the earliest branch of animals, a new study relied on an unlikely feature: the organization of genes into chromosomes. Despite having evolved separately for almost 500 million years, the chromosomes of sponges, jellyfish, and many other invertebrates share comparable sets of genes.

The chromosome structure of ctenophores was unknown until 2021 when Schultz and his co-advisers determined the chromosome structure of the ctenophore Hormiphora californiensis, which looked very different from those of other animals. This suggests that the slow rate of change in the chromosomes of many animals made it possible for scientists to use computational methods to recreate the chromosomes of the common ancestor.

The researchers collaborated to sequence the genomes of three unicellular organisms outside the animal kingdom, another comb jelly, and a sponge.

Comparing the chromosomes of these diverse animals and non-animals revealed that ctenophores and non-animals shared certain gene-chromosome combinations. In contrast, the chromosomes of sponges and other animals were rearranged in a distinct manner. The most straightforward explanation is that ctenophores branched off prior to the rearrangements.

This research will help us understand the fundamental functions we all share, such as how they perceive their environment, eat, and move. Rokhsar emphasized that five sets of gene-chromosome combinations firmly support the team’s findings. The only way the alternative sponge-first hypothesis could be true is if both sponges and non-ctenophore animals experienced multiple convergent rearrangements, which is extremely unlikely.

Related Publication

Schultz, D. T., S. H. D. Haddock, et al. (2023). “Ancient gene linkages support ctenophores as sister to other animals.” Nature


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