New research into chromosome structure sheds light on the earliest branches of the animal family tree. It suggests comb jellies, known as ctenophores, diverged first, with sponges following, before all other animal lineages, including the one that led to humans.
Even though ctenophores split from sponges early in evolutionary history, both groups continued to diversify from their shared ancestor. Scientists believe these two lineages retain traits from the earliest animals. Studying these ancient branches helps explain how animals originated and diversified into the wide array of forms seen today.
The closest common ancestor of all animals likely lived between 600 and 700 million years ago. It was probably soft-bodied and left little direct fossil evidence, making reconstruction challenging. Still, comparing living species provides clues about that ancestral stage, as explained by researchers from the University of Vienna in Nature.
relationships between lineages
Understanding how different animal groups relate to one another illuminates how key bodily features evolved, including the nervous system, muscles, and the digestive tract, the researchers note.
Until recently, sponges were thought to be the oldest animals.
“We’ve developed a new way to gain deep insights into the origins of animal life,” said the study’s lead author, emphasizing that these findings create a foundation for the scientific community to more accurately understand how animals evolved.
Most familiar animals—worms, flies, mollusks, starfish, and vertebrates like humans—share a body plan with a head containing a central brain and a gut running from mouth to anus, with muscles and other common features. This bilateral symmetry began taking shape around the time of the Cambrian Explosion, roughly 500 million years ago.
But other lineages such as jellyfish, sea anemones, sponges, and ctenophores display simpler body designs. They often lack a defined brain or a nervous system or muscles, yet they still demonstrate the fundamental development of multicellular organisms from a fertilized egg.
The order in which these lineages branched from the main trunk of the animal family tree has long been debated among scientists.
first animals discovered
DNA sequencing enabled researchers to compare gene sequences shared across animals and construct a family tree showing how animal life and its genes evolved since the Precambrian era. Yet gene-based methods have not settled the question of whether sponges or ctenophores mark the earliest branch, partly due to the deep antiquity of their differences, according to Rokhsar and colleagues.
Sponges appear primitive at first glance. After a free-floating larval stage, they settle and filter small food particles from seawater through their pores. They lack nerves and muscles, yet their structural features have practical uses in human life, despite their simple physiology.
Traditionally, sponges have been viewed as the oldest surviving animal lineage. Some scientists note that sponges lack a nervous system and muscles and resemble simple colonial forms of single-celled organisms.
The sponge lineage holds onto many features from the early animal ancestor that led to all other animals, including the precursors to neurons, nerves, muscles, and guts. Sponges look simple because they do not possess these later-developing features.
Another group proposed as the oldest animal lineage is the honeycomb jellyfish cluster. These creatures are popular in aquariums. While superficially jellyfish-like, they are distinct and often have two lobes or a bell shape with tentacles. Ctenophores move through water with eight rows of cilia arranged in comb-like patterns.
Organization of genes in chromosomes
To determine whether sponges or ctenophores were the earliest branch, the new study examined a lesser-known feature: the organization of genes along chromosomes. Each species has a characteristic chromosome count and a distinctive gene distribution along those chromosomes.
Earlier work showed that the chromosomes of sponges, jellyfish, and many other invertebrates carry similar sets of genes despite more than half a billion years of separate evolution. This slow chromosomal evolution allowed researchers to computationally reconstruct the chromosomes of the common ancestor across different animal lineages.
Chromosome-level structure of ctenophores remained unknown until 2021 when Schulz, then a graduate student at UC Santa Cruz, along with advisors Richard Green and Steven Haddock, mapped the chromosome structure of Hormiphora californensis. The findings highlighted a genome arrangement that stood in stark contrast to other major animal groups.
sequencing genomes
Researchers then sequenced the genomes of another ctenophore and a sponge, along with three single-celled relatives outside the animal lineage: a choanoflagellate, a philasterean amoeba, and a fish parasite called an ichthyospore. While approximate genome sequences existed for these non-animals, they did not provide the chromosome-scale gene placement needed to draw firm connections.
Comparisons across these taxa revealed striking patterns: ctenophores and non-animal lineages shared certain gene and chromosome arrangements, while sponges and other animals displayed a quite different organization. This pattern suggested that ctenophores diverged before major chromosome rearrangements occurred in the animal lineage, supporting the idea that they represent one of the earliest branches in the animal family tree.
These results offer a strong, if nuanced, view of early animal evolution and underscore the value of chromosomal-level data in resolving long-standing debates about the tree of life. Attribution: findings summarized from research reported in Nature, with researchers including Schultz and Simakov among others, reflecting a collaborative approach to understanding animal origins.
Reference work: Nature article on early animal evolution and chromosome organization.
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