Imagine a photograph of your great-grandparents, grandparents and parents side by side. You’d see a resemblance, but each generation would look distinct from its predecessors. This is the process of evolution in its simplest form: descent with modification.
Over many generations, a staggering amount of modification is possible. This is how the diversity of life on Earth came to be.
This idea, though, has long been misunderstood as a path that leads in one direction toward “higher” or “better” organisms. For example, Rudolph Zallinger’s famous 1965 Time-Life illustration “The Road to Homo Sapiens” shows humans evolving in a stepwise fashion from ape-like ancestors to modern man.
Extending this perspective beyond humans, early paleontological theories about ancient life supported the idea of orthogenesis, or “progressive evolution,” in which each generation of a lineage advanced toward more sophisticated or optimized forms.
But evolution has no finish line. There is no end goal, no final state. Organisms evolve by natural selection acting at a specific geologic moment, or simply by drift without strong selection in any direction.
In a recently published study that I carried out with Makaleh Smith, then an undergraduate research intern at Harvard University who was funded by the National Science Foundation, we sought to study whether a one-way model of reproductive evolution always held true in plants. To the contrary, we found that in many types of ferns — one of the oldest groups of plants on Earth — evolution of reproductive strategies has been a two-way street, with plants at times evolving “backward” to less specialized forms.
Related: Which animals are evolving fastest?
The path of evolution is not linear
Selection pressures can change in a heartbeat and steer evolution in unexpected directions.
Take dinosaurs and mammals, for instance. For over 150 million years, dinosaurs exerted a strong selection pressure on Jurassic mammals, which had to remain small and live underground to avoid being hunted to extinction.
Then, about 66 million years ago, the Chicxulub asteroid wiped out most nonavian dinosaurs. Suddenly, small mammals were relieved of their strong predatory selection pressure and could live above ground, eventually evolving into larger forms, including humans.
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