3.8–3.5 Billion Years Ago: Estimated Beginnings of Life on Earth
This estimate may change over time as new evidence becomes available. It is thought that the first life developed in undersea vents and was RNA, rather than DNA-based. 1
Additionally, around this time, a common ancestor gave rise to two groups of life: bacteria and archaea. The mechanism by which this happened and the exact time at which this split occurred is still under investigation.
The oldest fossils of single-celled organisms date back 3.5 billion years and present with metabolic pathways analogous to those of recent prokaryotic organisms. 2
2.4 Billion Years Ago: Great Oxidation Event
It is believed that the waste produced by cyanobacteria, oxygen, began to build up in the atmosphere around this time.
2 Billion Years Ago: Eukaryotic Cells Arise
At this time, eukaryotic cells—cells with internal “organs” called organelles—came into being. The key organelle here is the nucleus, where genes are stored in the form of DNA.
These cells came about when one simple cell engulfed another via endosymbiosis. These same endosymbiotic events are used to explain the existence and function of chloroplasts in plant cells and mitochondria in animal cells.
900 Million Years Ago: First Multicellular Life Develops
Though the mechanism by which this occurred is still under investigation, it has been hypothesized that single-celled organisms formed colonies of many individuals, eventually making multicellularity possible.
465 Million Years Ago: Plants Colonize Land
Plants colonized land because this move provided a range of advantages to plant life: brighter sunlight, more abundant carbon dioxide, and soil rich with nutrients.
Because early land plants did not yet have a mechanism for transporting water throughout their bodies, however, they relied on wet environments. These early plants are known as bryophytes: mosses, liverworts, and hornworts. 3
With time, plants became more advanced with ferns, gymnosperms like pine trees, and finally angiosperms, the flowering plants we know today.
Like humans, plants experienced their own evolutionary timeline that allowed them to become better-suited for their environments. One of these eventual adaptations were secondary metabolites, which were not necessary for basic plant metabolism but allowed for long-term survival. 4
Later, it became clear that these secondary metabolites, like terpenes, are released into the air and benefit surrounding plants and other organisms who breathe the terpene-rich air.
65 Million Years Ago: Extinction of the Dinosaurs
The cause of the mass extinction that wiped out the dinosaurs remains a scientific mystery. Scientists have narrowed the explanation down to two hypotheses: the strike of an asteroid or comet, or unusually high rates of volcanism. Either scenario would have blocked the light of the sun, killing plant life and disrupting the food chain all the way up to the dinosaurs.
The extinction of the dinosaurs in particular highlights the importance of plant life to the survival of organisms. As primary producers, plants set up the success of the rest of the ecosystem. Without them, the rest of the food chain collapses. 5
6 Million Years Ago: Humans Diverge from Ancestors
About six million years ago humans diverged from their closest ancestors, the chimpanzees, and the bonobos. Soon afterwards, they began walking on two legs. The rest is history.