The question of what the first animal looked like concerns the origin of Metazoa, the group that includes all multicellular animals. This event marks one of the most profound transitions in the history of life, where simple colonial life gave rise to complex organisms with differentiated cells. Looking back over 600 million years, the fossil record offers only glimpses. Scientists must rely on comparative genomics and morphology to reconstruct this ancient ancestor, whose identity and appearance remain intensely debated in evolutionary biology.
The Jump to Multicellularity
The first animal evolved from a specific line of single-celled organisms, transitioning from individual cells to a coordinated community. This ancestor is thought to have been similar to modern-day choanoflagellates, which are the closest living relatives to animals. Choanoflagellates are small aquatic protists that live as solitary individuals or form simple, temporary colonies.
The choanoflagellate cell uses a single flagellum surrounded by a collar of microvilli to filter-feed. This structure is nearly identical to the choanocyte cells found inside sponges, providing strong morphological evidence for their close relationship. The genetic toolkits for cell adhesion and signaling, fundamental to animal life, were already present in these unicellular ancestors. The evolutionary leap involved repurposing these pre-existing genes to coordinate the activities of multiple cells into a stable, functional organism.
The Leading Candidates for the Earliest Animal Lineage
Debate over the appearance of the first animal focuses on which modern lineage branched off first from the ancestral stock. The two main contenders for the most basal animal group are the sponges (Porifera) and the comb jellies (Ctenophora). For decades, the simple body plan of the sponge made it the favored candidate.
Sponges lack true tissues, organs, and a nervous system, functioning instead as loose assemblies of specialized cells, including choanocytes. This simple, sedentary, filter-feeding organization aligns with the expectation that the first animals were anatomically rudimentary. If sponges are the most basal group, the first animal would have looked like a simple, porous, non-motile sac attached to the seafloor, relying on water currents for survival.
The comb jellies, or ctenophores, present a complex challenge to this view. These free-swimming, gelatinous marine predators possess sophisticated features such as a nervous system, muscles, and distinct tissue layers. Recent genomic analyses suggest that the comb jellies were the first to diverge from the rest of the animals. If this “ctenophore-first” hypothesis is correct, it implies the first animal was unexpectedly complex, meaning sponges must have secondarily lost features like neurons and muscles over time.
How Scientists Date the First Animals
Determining when the first animal appeared involves reconciling two lines of evidence: the fossil record and molecular clocks. The fossil record provides tangible, though scarce, evidence, with the earliest widely accepted animal fossils appearing in the Ediacaran biota, roughly 580 to 541 million years ago. These fossils mostly represent later, more complex forms of life, and the soft-bodied nature of the very first animals meant they rarely preserved well enough to be found.
Molecular clocks offer an independent method for dating evolutionary divergence. This technique uses the rate at which genetic mutations accumulate in the DNA of living species to estimate when they split from a common ancestor. Molecular clock analyses consistently push the origin of Metazoa much further back in time, often placing the initial divergence between 700 and 800 million years ago in the Cryogenian or Tonian periods.
The large gap between the fossil evidence and the genetic estimates highlights a conflict in the data. The molecular data suggest a long, unseen period of “cryptic” animal evolution, where small, soft-bodied animals existed but left no substantial trace. The first animals were likely small and simple enough to be missed in the rock record until they evolved the size and hard parts seen in the later Ediacaran and Cambrian periods.
Reconstructing the Last Common Ancestor of All Animals
Despite the debate over the most basal modern lineage, scientists have formed a cohesive picture of the hypothetical Last Common Ancestor of all Animals (LCA). This organism must have possessed the minimal features shared by all subsequent animal phyla. It was a small, simple organism, likely measuring only a few millimeters in size.
The LCA was probably non-motile, or only capable of limited movement, and lacked a defined gut or internal organs. It would have been asymmetrical or radially symmetrical, functioning as a suspension feeder that filtered tiny particles from the water column. Its physical appearance was likely that of a hollow, sac-like organism or a flattened, two-layered blob, representing a coordinated community of specialized cells.