Life › Complex cells & first multicellular life › Ediacaran biota
First animals
by c. 650–600 million years ago (sponge-like; debated)· Organism or group· Animals
DebatedScientists disagree about when, how or why.
The earliest animals were probably simple sponge-like filter feeders, appearing sometime around 650 to 600 million years ago or earlier.
See it on the Life map →What happened
Animals are many-celled organisms that eat other organisms and develop from embryos. Their closest single-celled relatives, choanoflagellates, look like the feeding cells of sponges. Molecular clocks put the first animals roughly 800–650 million years ago. Possible chemical traces of sponges appear in Oman rocks older than 635 million years. Widely accepted animal fossils, such as Dickinsonia and Kimberella, appear only around 560 million years ago.
Why it matters
Animals added grazing, filter feeding, burrowing and later predation, reshaping seafloors and ocean chemistry and setting up the Cambrian explosion.
The evidence (fossils, genes, rocks)
Gene-based molecular clocks, sponge-type steroid molecules in rocks older than 635 million years, contested microfossils, and the first clear body fossils in Ediacaran rocks.
Still debated
The sponge chemicals might come from algae or later alteration. Whether sponges or comb jellies branched off first is also hotly debated.
Connections (2)
▸Multicellular life arose again in First animalsStrong evidence
The connection
Animals are a separate invention of multicellularity, starting from single-celled ancestors like today's choanoflagellates, some of which form colonies. Animal cells added glue proteins such as collagen, put older adhesion genes such as cadherins to new use, and built new signaling systems, allowing specialized cells and embryos that develop in stages.
When and how
Probably around 800–650 million years ago in the oceans; Nicole King's lab compared choanoflagellate and animal genomes from 2008.
How sure are we?
Choanoflagellates carry many genes once thought unique to animals, and some of their colonies form by cell division, much like an early embryo.
Debates and open questions
Whether the first animals resembled sponges, comb jellies or something else, and why animal multicellularity came so late.
▸Dickinsonia is early chemical evidence of First animalsStrong evidence
The connection
Dickinsonia was not the first animal, but it is among the earliest whose animal identity is backed by chemical evidence. Its fossils preserve cholesterol-type steroids that animals make, showing that large animals existed by about 558 million years ago, after decades of doubt.
When and how
White Sea coast of northwestern Russia, about 558 million years ago; Ilya Bobrovskiy and colleagues published the result in Science in 2018.
How sure are we?
The organic film was dominated by cholesteroids, unlike the algal and microbial sterols in surrounding mat material.
Debates and open questions
A minority argues the steroids could come from decaying mats or other sources; where Dickinsonia branches among animals is unresolved.