The search for Earth's very first animal remains a scientific debate

Science · Published 13 October 2026 Swipe more like this →

In short: Long before blue whales or even microscopic tardigrades, life on Earth was a different world. While we know microbes existed 3.7 billion years ago, identifying the first true animal is a mystery involving a clash between fossil evidence and genetic dating.

The history of life is a transition from simple to complex. For much of Earth's early history, the environment was high in methane and lacked the oxygen necessary for most modern animals. The earliest signals of life are found in 3.7 billion-year-old rocks, specifically through carbon molecules produced by microbes. By 3.5 billion years ago, these microbes created stromatolites—layered structures formed as sticky microbial mats trapped sediment.

A massive shift occurred at least 2.4 billion years ago with the evolution of cyanobacteria. As the planet's first photosynthetic organisms, they used sunlight and water to create food, releasing oxygen as a byproduct. This 'Great Oxidation Event' is visible in Banded Iron Formations (BIFs) on the seafloor, where oxygen reacted with iron-rich water to create iron oxide minerals. However, oxygen levels fluctuated; the decomposition of dying cyanobacteria likely caused oxygen levels to drop again, meaning the oceans remained difficult for oxygen-dependent life for billions of years.

The leap to animal life required the development of eukaryotic cells—complex cells with nuclei and specialized organelles like mitochondria, which evolved from microbes living inside other microbes. This cellular complexity allowed for multicellularity, where groups of cells could cooperate for protection or better feeding. Today, paleontologists and geneticists are divided on which complex organism holds the title of the first animal, with the debate primarily narrowing down to two candidates: sponges and comb jellies.

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