What Defines an Animal?
Animals, formally known as Metazoa, are a diverse kingdom of multicellular, eukaryotic organisms. A primary characteristic is heterotrophy, meaning they obtain nutrients by consuming other organisms.
Animals generally lack rigid cell walls, which allows for flexibility and motility, key features for many animal lifestyles. Most animals also develop from a blastula, a hollow ball of cells formed during embryonic development. While simpler animals may not possess all complex features, the presence of specialized cells and tissues, such as nerve and muscle cells, further defines this group, enabling coordinated movement and responses to their environment. These shared biological traits are crucial for scientists when trying to identify the earliest branches on the animal family tree.
Tracing Life’s Earliest Stages
Animal life emerged after billions of years of microbial evolution. Life first appeared on Earth approximately 3.85 billion years ago as simple, single-celled organisms called prokaryotes. Later, more complex life forms, eukaryotes, evolved, characterized by cells with a nucleus and other membrane-bound organelles. This evolutionary progression eventually led to the development of multicellularity, a key innovation where individual cells began to cooperate and specialize.
The transition to multicellularity occurred independently multiple times across different lineages, but its development was a prerequisite for the complexity seen in animals. The first multicellular animals are thought to have appeared much later, during the Neoproterozoic Era, specifically in the Ediacaran Period (635 to 541 million years ago), long before the Cambrian Explosion. This period saw the diversification of early complex organisms, setting the stage for a significant increase in animal forms that would follow.
The Leading Contenders for the First Animal
The first animal on Earth is a subject of ongoing scientific debate, with several strong candidates based on their simple body plans and genetic analyses. Sponges (Porifera) have long been viewed as a prime contender due to their apparent simplicity, lacking true tissues and organs. Chemical fossils, specifically a steroid molecule called 24-isopropylcholestane (24-ipc) found in 640-million-year-old rocks, provide molecular evidence supporting the presence of sponges predating the Cambrian Explosion. This molecular signature aligns with genetic studies that place sponges very early on the animal evolutionary tree.
However, recent genetic studies have presented a compelling case for comb jellies (Ctenophora) as the earliest branching animal lineage. These marine invertebrates possess more complex features than sponges, including nerve cells and muscle-like cells. Genomic analyses suggest that comb jellies diverged before all other animals, including sponges. This implies that some of their complex features might have evolved independently or were lost in other early lineages.
Placozoans, tiny, flat, amoeba-like marine invertebrates, also feature in discussions about early animal evolution due to their extreme structural simplicity. They lack a fixed body shape, organs, and a nervous system, relying on only a few cell types. While their exact phylogenetic position remains debated, some molecular analyses suggest they are among the most basal animals, possibly even predating the split between cnidarians and bilaterians.
Unraveling the Mystery: Scientific Approaches and Challenges
Scientists use various approaches to investigate animal origins, but no single, definitive answer exists. The study of ancient fossils provides direct evidence of past life, with discoveries like the Ediacaran biota revealing early complex multicellular organisms. However, the fossil record of early, soft-bodied animals is often incomplete and difficult to interpret, as these organisms rarely preserve well. The fragile nature of early animal bodies means that many ancestral forms likely left no fossil trace.
Comparative genomics and molecular phylogenetics offer another powerful tool, using DNA similarities to reconstruct evolutionary relationships and estimate divergence times. These “molecular clocks” can suggest that animal lineages originated much earlier than indicated by the fossil record, sometimes by over 100 million years. Developmental biology also contributes insights by examining how modern animals develop, providing clues about the evolutionary changes that occurred in their ancestors.
Despite these advanced methods, reconciling molecular clock estimates with the scarce fossil evidence remains a significant challenge. The rapid evolutionary changes that characterized early animal diversification, coupled with the difficulty in distinguishing true animals from other multicellular organisms in the earliest records, further complicate the picture. The search for the first animal is an active and complex area of research.