The identity of the first animal on Earth is still debated among scientists, but two main candidates emerge from current research: sponges and comb jellies (ctenophores).

• Sponges have long been considered among the earliest animals. Chemical compounds linked to sponges have been found in rocks as old as 700 million years, and molecular evidence suggests they may have developed even earlier. Sponges are simple, sessile filter feeders that tolerate low oxygen conditions, which fits well with early ocean environments. They also have a simple body plan and are thought to be the first reef builders on Earth

• Comb jellies (ctenophores) are another strong candidate. A 2023 study using chromosomal data from modern comb jellies argues that they were the first known animals, emerging around 600 to 700 million years ago. Comb jellies are more complex than sponges, possessing muscles and nervous systems, but their earliest ancestors might have been simpler. Genetic evidence from chromosome organization suggests comb jellies branched off first from the animal lineage, before sponges and all other animals

Fossil evidence is scarce and ambiguous for animals older than about 570 million years. The earliest widely accepted animal fossils date from the Ediacaran period (~571 million years ago) and include soft-bodied creatures, but these may not represent the very first animals

. In summary, the first animal on Earth was likely a simple multicellular organism resembling either a sponge or a comb jelly, appearing roughly 600 to 700 million years ago. Current genetic evidence slightly favors comb jellies as the earliest branch of the animal kingdom, but sponges remain a strong contender based on molecular fossils and chemical signatures in ancient rocks