The question of whether plants or animals came first is a fundamental inquiry into the history of life on Earth. The answer is not a simple choice, as both modern plants and animals evolved from a much simpler, shared ancestor. Tracing the evolutionary timeline reveals that the two kingdoms split from a common single-celled predecessor that was neither plant nor animal. Understanding which came first requires examining the earliest forms of life before complex multicellularity arose.
The Dawn of Life Before Plants and Animals
Life on Earth began approximately 3.8 billion years ago, long before the appearance of anything recognizable as a plant or an animal. The earliest organisms were single-celled microbes called prokaryotes, including bacteria and archaea. These simple cells thrived in an atmosphere with virtually no free oxygen, relying on anaerobic metabolism.
A revolutionary change occurred with the evolution of cyanobacteria, prokaryotes capable of oxygenic photosynthesis, around 2.7 billion years ago. These organisms began releasing oxygen as a byproduct, slowly accumulating it in the oceans and atmosphere. This process, known as the Great Oxygenation Event, prepared the planet for the eventual evolution of more complex, oxygen-dependent organisms.
The Common Ancestor of Plants and Animals
The evolutionary leap from simple prokaryotes to the more complex cells found in plants and animals occurred with the emergence of eukaryotes. Eukaryotic cells are defined by having a membrane-bound nucleus and specialized internal compartments called organelles. This major transition is thought to have happened around 1.5 to 2 billion years ago, setting the stage for all complex life.
A defining event in the formation of eukaryotes was endosymbiosis, where one cell engulfed another, forming a mutually beneficial relationship. Mitochondria, found in all plant and animal cells, are the descendants of ancient bacteria incorporated into a larger host cell. The lineage leading to both plants and animals began with this single-celled ancestor, classified today as a type of protist.
The two kingdoms diverged from this protist ancestor, which possessed a nucleus and mitochondria. This common ancestor was neither a plant (lacking photosynthesis) nor an animal (not a complex, mobile, heterotrophic organism). The split occurred when the animal ancestor maintained its heterotrophic lifestyle, while the plant ancestor acquired a second endosymbiont.
The Evolutionary Path to Animal Life
The animal lineage, known as Animalia, is characterized by heterotrophy—the need to consume other organisms for energy—and the eventual development of complex mobility. The path to the first true animals began with single-celled organisms like choanoflagellates, considered the closest living relatives of animals. Choanoflagellates resemble the collar cells found in modern-day sponges, which are the earliest known animals.
The earliest forms of multicellular animal life appeared during the Ediacaran Period, between 635 and 541 million years ago. These soft-bodied organisms, known as the Ediacaran biota, were mostly immobile and filter-feeding. More recognizable animal forms, including those with bilateral symmetry and hard parts, exploded in diversity during the subsequent Cambrian period, which began approximately 541 million years ago.
The Evolutionary Path to Plant Life
The plant lineage, Plantae, became distinct when its protist ancestor underwent a second endosymbiotic event. This involved the engulfment of a cyanobacterium, which evolved into the chloroplast, the organelle responsible for photosynthesis. This acquisition allowed the lineage to become photoautotrophic, creating its own food from sunlight.
For hundreds of millions of years, the plant lineage existed as various forms of green algae in aquatic environments. The transition from water to land occurred much later, roughly 470 million years ago, when early plants evolved structures like a waxy cuticle to prevent desiccation. This move onto dry land, long after the first animals had appeared in the oceans, allowed plants to exploit a new environment. Therefore, while the common ancestor was a protist, the first true animals in the fossil record predate the first true land plants.