The question of the first animal to exist on Earth is a complex puzzle in evolutionary biology, marking the moment single-celled existence gave way to the complex world of animals. The answer is an ongoing scientific debate over which lineage first branched from the tree of life, an event that occurred hundreds of millions of years before recognizable hard-shelled creatures appeared. Scientists rely on conflicting evidence, including the incomplete fossil record and the deep history locked within the genomes of living species, to reconstruct this long-lost ancestor. Resolving this deep evolutionary split is fundamental to understanding how defining features of animal life, from nervous systems to muscles, came to be.
Defining the Earliest Animal
The term “animal,” or Metazoa, is defined by specific biological characteristics that distinguish it from the single-celled organisms, or protists, that preceded it. A primary feature is obligate multicellularity, where cells depend on each other for survival and are organized into a cohesive whole. This cellular organization enables a division of labor, specializing cells for functions like feeding, movement, or reproduction.
This specialization is supported by a molecular toolkit, including proteins for cell-to-cell adhesion and communication. Unlike plants or fungi, animals are heterotrophs, meaning they obtain nutrients by consuming other organisms rather than producing their own food. Developmentally, nearly all animals begin life as a zygote that forms a hollow sphere of cells known as a blastula, a defining stage in the animal life cycle.
The Leading Evolutionary Candidates
The search for the basal, or earliest diverging, animal lineage centers on three disparate groups: Porifera (sponges), Ctenophora (comb jellies), and Placozoa. Sponges were traditionally considered the first animals to branch off, supported by their morphological simplicity and lack of true tissues or organs. They are sessile, filter-feeding organisms whose cells resemble choanoflagellates, the closest single-celled relatives of animals. This “sponge-first” view suggests the last common animal ancestor was a simple, colonial-like organism, with subsequent animal complexity evolving later.
Modern genomic analysis has challenged this view by frequently placing Ctenophora, or comb jellies, as the sister group to all other animals. Ctenophores are gelatinous, free-swimming marine predators that possess greater complexity, including a nerve net and specialized muscle cells. If comb jellies are the earliest branch, it implies a complex evolutionary path. This suggests either the last common ancestor already possessed a nervous system and muscles, which were secondarily lost in sponges, or these complex traits evolved independently in the ctenophore lineage. Recent studies focusing on genomic characters, such as chromosome arrangement, strongly support the “ctenophore-first” hypothesis.
Placozoa represents a third, less prominent candidate for the basal position, recognized for its extreme structural simplicity. Placozoans are millimeter-sized, amoeba-like marine creatures with only a few thousand cells organized into three distinct layers. They lack a mouth, nervous system, or any organs. Their simplicity has been interpreted by some as a strong ancestral character, suggesting they might resemble the earliest animal form. However, other analyses suggest their simplicity is a result of secondary loss and reduction from a more complex ancestor.
How Scientists Determine the Origin
Resolving the deep evolutionary split requires scientists to combine evidence from two different fields: paleontology and molecular biology. Paleontologists search the rock record for physical evidence, categorized into body fossils, trace fossils, and biomarkers. Body fossils of the earliest animals are exceptionally rare because these soft-bodied creatures left only faint impressions, such as those found in the Ediacaran biota.
Trace fossils, including burrows, trails, and feeding marks, provide indirect evidence of behavior and movement, often predating the body fossils. The earliest and most contentious evidence comes from chemical signatures, or biomarkers, which are specific organic molecules preserved in ancient rocks. For example, certain sterane molecules characteristic of modern sponges have been found in rocks dating back 650 million years ago, suggesting sponge-like organisms existed well before the Ediacaran body fossils.
Molecular biologists use a technique called the molecular clock to estimate divergence times by analyzing genetic differences between living species. This method assumes that neutral mutations in DNA accumulate at a relatively constant rate over millions of years. By comparing mutations and calibrating the rate using fossil dates, researchers project the time when groups last shared a common ancestor. This technique has estimated the split of major animal groups to between 600 and 800 million years ago, but results vary significantly depending on the genes and calibration points used.
The Ancient Timeline of Animal Life
The first animals are thought to have emerged during the Neoproterozoic Era, spanning the late Cryogenian and Ediacaran periods (roughly 800 to 540 million years ago). This period precedes the Cambrian explosion, which saw the sudden appearance of most modern animal body plans around 539 million years ago. The earliest hints of metazoan life are chemical, with sponge-specific lipid biomarkers suggesting an origin of simple animal forms around 650 million years ago.
The first widely recognized physical evidence of complex multicellular life belongs to the Ediacaran biota, which thrived between 580 and 540 million years ago. These organisms were soft-bodied, often disc-shaped or frond-like, and their relationship to modern animal phyla remains ambiguous. During this time, basic animal characteristics, such as specialized cells and simple body plans, were established, setting the stage for rapid diversification.