The common animal ancestor, or the last universal common ancestor (LUCA), is the hypothetical organism from which all living animals are descended. It is estimated to have lived about 4 billion years ago, and its features can be inferred from the shared characteristics of modern animals. However, the exact identity and phylogenetic position of LUCA is still a matter of debate among scientists.
Sponges, cnidarians, and ctenophores
Among the living animals, three groups are considered to be non-bilaterian, meaning that they do not have a bilateral symmetry or a distinct head and tail. These are sponges, cnidarians, and ctenophores. Sponges are simple animals that lack true tissues and organs, and feed by filtering water through their porous bodies. Cnidarians are aquatic animals that have radial symmetry and tentacles with stinging cells called cnidocytes. They include jellyfish, sea anemones, corals, and hydra. Ctenophores are also aquatic animals that have radial symmetry and rows of cilia that they use for swimming. They are also known as comb jellies or sea gooseberries.
According to the most recent molecular phylogenetic studies, the most closely related group to LUCA among these three is ctenophores. This means that ctenophores diverged from LUCA before sponges and cnidarians, and represent the earliest branching animal lineage. This hypothesis is supported by several lines of evidence, such as:
- Ctenophores have a unique set of genes that are markedly different from those of other animals, suggesting a long evolutionary distance from LUCA.
- Ctenophores have a nervous system that is distinct from that of other animals, and may have evolved independently from LUCA.
- Ctenophores have a complete digestive tract with a mouth and an anus, which is absent in sponges and cnidarians.
- Ctenophores lack some features that are shared by sponges and cnidarians, such as collagen, cell junctions, and Hox genes.
However, this hypothesis is not universally accepted by all scientists, and some alternative hypotheses have been proposed, such as:
- Sponges are the most closely related group to LUCA, and ctenophores and cnidarians are more derived.
- Sponges and ctenophores are equally related to LUCA, and cnidarians are more derived.
- All three groups are equally related to LUCA, and represent a polytomy (a branch point with more than two descendants).
The resolution of this debate requires more data and analysis from different sources, such as fossils, morphology, development, and ecology.
The question of which of the following is thought to be the most closely related to the common animal ancestor is not an easy one to answer. The current evidence suggests that ctenophores are the most likely candidates, but this is not a definitive conclusion. More research is needed to clarify the evolutionary relationships among sponges, cnidarians, ctenophores, and LUCA.