The question of the first animal discovered on Earth concerns which lineage branched off earliest in the tree of life. Scientists are working to uncover the identity of the organism that represents the common ancestor of all animals, a quest that requires both geological evidence and advanced genetic analysis. Resolving this mystery sheds light on how complex features like muscles and nervous systems originally evolved.
Defining the Earliest Animals
To identify the first animal, scientists must first establish a definition for what constitutes an animal, or Metazoan. Animals are defined by fundamental traits that separate them from other life forms like plants and fungi. A primary characteristic is multicellularity, where an organism is composed of multiple cells that cooperate, often with specialized functions.
Animals are also heterotrophs, meaning they obtain nutrients by consuming other organisms rather than producing their own food. This consumption is typically achieved through ingestion, distinguishing them from fungi which absorb nutrients. Furthermore, animal cells lack the rigid cell walls found in plants and fungi, allowing for greater flexibility and mobility.
The earliest forms may have been much simpler than modern animals. Even the most basic animals exhibit a degree of cellular organization, and many have a motile phase, meaning they can move during at least one stage of their life cycle. The first animal must meet these minimal criteria, even if it lacked specialized tissues and organs.
Evidence from the Deep Past: The Fossil Record
The most direct evidence for early life comes from the fossil record, but soft-bodied organisms, like the first animals were likely to be, rarely fossilize well. The oldest known complex, multi-celled life forms are found in the Ediacaran biota, a collection of enigmatic organisms that appeared roughly 635 to 538 million years ago. These fossils, which often resemble quilted disks or fronds, present a challenge because their body plans do not clearly align with modern animal groups.
For decades, the Ediacaran biota was a biological puzzle. However, the discovery of molecular fossils provided a breakthrough. Chemical analysis of the Ediacaran fossil Dickinsonia revealed the presence of cholesteroids, a type of steroid compound unique to animals.
This molecular signature confirms that Dickinsonia, which lived around 558 million years ago, was a true animal, establishing a minimum age for the existence of macroscopic animals. The earliest animal ancestor must have existed even earlier, because the Ediacaran period already shows a diversification of forms. The fossil record primarily shows when a lineage became large enough to be preserved, not its actual point of origin.
Molecular Clocks and Genetic Divergence
To look further back in time than the fossil record allows, scientists use the molecular clock technique. This method compares the number of genetic differences, or mutations, in the DNA sequences of different species, assuming mutations accumulate at a relatively constant rate over geological time.
By calibrating this “clock” using known divergence dates from the fossil record, researchers estimate when any two groups last shared a common ancestor. Molecular clock analyses consistently indicate that the common ancestor of all animals lived far earlier than the Cambrian Explosion, potentially between 700 and 900 million years ago.
The application of the molecular clock has been complicated because evolutionary rates vary significantly between different lineages. Early genomic studies yielded conflicting results, leading scientists to develop more sophisticated analytical methods. Genetic analysis is useful for establishing the order in which major animal phyla diverged from the common ancestor, charting the earliest branches on the animal family tree.
The Leading Evolutionary Candidates
The evidence from both fossils and genetics has narrowed the search for the first animal down to two primary contenders: sponges (Phylum Porifera) and comb jellies (Phylum Ctenophora). Sponges were traditionally considered the earliest branch due to their morphological simplicity, lacking true tissues, muscles, and a nervous system. This view aligns with the idea that complexity evolved gradually.
However, genomic analysis surprisingly suggested that comb jellies, which possess muscles and a rudimentary nervous system, were the first to diverge. This finding implied that the earliest animal was more complex and that sponges either lost these features or evolved them independently, sparking a significant scientific debate.
More recent and robust studies using large genomic datasets have since provided strong support for the traditional view, placing sponges at the base of the animal family tree. The current consensus suggests that the simple, filter-feeding sponges represent the oldest living animal lineage. This conclusion reaffirms that the earliest steps in animal evolution involved an organism with a basic, tissue-less body plan.