The question of whether plants or animals came first on Earth seems straightforward, yet its answer unfolds a complex and captivating evolutionary saga. It is not a simple matter of one preceding the other in the way we recognize them today. The journey of life on our planet reveals a deep history, where fundamental forms emerged, diversified, and gradually gave rise to the intricate organisms that now populate diverse ecosystems. Understanding this timeline requires a look back to the very origins of life and the major evolutionary milestones that shaped its trajectory.
The Dawn of Life
Life on Earth began with simple, microscopic organisms billions of years ago. The earliest undisputed evidence points to prokaryotes, such as bacteria and archaea, appearing between 3.5 and 3.8 billion years ago. These single-celled organisms lacked a nucleus and other complex internal structures, thriving in an anoxic environment where molecular oxygen was virtually absent. For over a billion years, these simple life forms were the sole inhabitants, forming microbial mats known as stromatolites.
A pivotal moment in Earth’s history was the Great Oxygenation Event, which began around 2.4 to 2.1 billion years ago. This change was driven by the evolution of cyanobacteria, a prokaryote capable of oxygenic photosynthesis, releasing oxygen as a byproduct. Initially, this oxygen reacted with iron in the oceans, forming vast banded iron formations. Eventually, free oxygen accumulated in the atmosphere. This atmospheric shift had a dramatic impact, as oxygen was toxic to many existing anaerobic life forms, leading to an extinction event.
The increasing oxygen levels set the stage for the emergence of eukaryotic cells, a more complex life form characterized by a nucleus and membrane-bound organelles like mitochondria. Eukaryotes originated through symbiogenesis, where one prokaryotic cell engulfed another, leading to a symbiotic relationship. The earliest eukaryotic cells appeared around 2.2 billion years ago, still single-celled organisms, neither plant nor animal in their modern forms.
The Great Evolutionary Split
From these early single-celled eukaryotes, life embarked on immense diversification. All eukaryotes, including animals, plants, and fungi, share a Last Eukaryotic Common Ancestor (LECA), which was a single-celled organism. This ancient common ancestor possessed features like a nucleus and mitochondria, establishing the basic blueprint for all subsequent eukaryotic life.
Over vast stretches of geological time, different lineages of these single-celled eukaryotes began to diverge. These divergences led to distinct supergroups, each representing a foundational branch on the tree of life. For instance, the Opisthokonta supergroup gave rise to animals and fungi, while the Archaeplastida supergroup includes the ancestors of modern plants. At this early stage, organisms within these nascent lineages remained single-celled, their genetic paths separating long before complex, multicellular forms appeared.
The acquisition of chloroplasts, organelles responsible for photosynthesis in plants, occurred through a separate endosymbiotic event, where an early eukaryotic cell engulfed a cyanobacterium. This event marked the lineage that would lead to green algae and land plants. Thus, the fundamental cellular machinery defining “plant-like” and “animal-like” lineages was established in single-celled ancestors, long before the evolution of large, visible organisms.
The Rise of Multicellularity
While evolutionary lineages diverged early, the development of large, complex organisms we recognize as plants and animals occurred much later. Multicellularity, the state of multiple cells working together, evolved independently multiple times across different eukaryotic groups. This innovation offered numerous advantages, such as increased size, specialized cell functions, and improved efficiency in resource acquisition and defense.
The transition from single-celled to multicellular life involved cells adhering and developing mechanisms for communication and coordination. Early forms of multicellular life began to appear in the fossil record, distinct from their single-celled predecessors. These initial multicellular organisms were relatively simple, lacking the intricate tissues and organs found in later forms.
For example, the earliest multicellular algae, considered ancestors of plants, appeared in the fossil record, indicating independent evolution of multicellularity within the plant lineage. Similarly, early animal-like multicellular organisms emerged, demonstrating that the capacity for complex body plans arose separately in these diverging groups. This independent development highlights that multicellularity was an advantageous strategy, leading to the explosion of diverse life forms.
Addressing the Timeline
Considering the vast evolutionary journey, neither “plants” nor “animals” in their familiar forms came first. Instead, single-celled ancestors of both groups existed for billions of years, diverging from a common eukaryotic ancestor. The foundational split between plant and animal lineages occurred when these organisms were still microscopic and unicellular, with the plant lineage gaining photosynthetic capabilities through a distinct event.
Complex, multicellular life forms emerged much later in Earth’s history. The earliest undisputed animal fossils, part of the Ediacaran biota, appeared approximately 575 to 540 million years ago. Around the same geological period, early complex algal forms, ancestors of modern plants, also became evident. Therefore, simple multicellular forms resembling early plants and animals appeared relatively close in geological time, following the evolutionary innovations of eukaryotic cells and multicellularity.
The evolutionary path was a gradual process of development and diversification from these shared ancestral forms. It was not a direct competition where one fully formed group preceded the other. Instead, life diversified from ancient single-celled organisms, with plant and animal lineages evolving multicellularity and complexity along parallel, yet distinct, trajectories.