The Tangled Tree

There’s a good chance that, at some point, you’ve seen a picture of “the tree of life.” It’s a drawing in the form of a tree that represents the evolution of one type of life, tracking progress from its “roots” as a primordial amoeba to a fish to an amphibian and beyond. At various points along the way, branches diverge from the primary trunk, representing worms, reptiles, rodents and other animals.

These illustrations have long served as a useful tool, a simple visual model of a complex subject.

Like many other things in science, the concept behind the tree of life diagram can be traced back to Aristotle. He mentioned the progressional development of animals in his book History of Animals, written in the fourth century BCE. However, Aristotle suggests that progress in nature is akin to ascending a ladder; living organisms start out as elements such as earth, water and fire, and gradually evolve into plants, animals and then humans. At the top of the evolutionary ladder, humans turn into heavenly beings. All life is part of one “stairway to heaven,” so to speak.

This model was popular for many centuries, with the “ladder of ascent” even being referenced in sixteenth-century woodwork. But by 1745, Enlightenment-era thinkers had started using more tree-like models.

At this time, explorers began seeing more of the world. Information and knowledge were spreading, and scholars needed more than a one-way ladder to classify all the diverse new plants and animals. A tree wasn’t so much a perfect expression of evolution as a handy way to categorize biological information.

As the French botanist Augustin Augier wrote in 1801, an illustrated “tree appears to be the most proper way to grasp the order and gradation” of plant life.

The tree of life may have reached its apex with the gifted illustrator and biologist Ernst Haeckel. In the latter half of the 1800s, Haeckel published multivolume books filled with remarkably detailed drawings of fascinating microscopic creatures and more than a few trees of life.

But unlike Augier’s tree, Haeckel drew evolutionary trees that illustrated the precise lineage of living things. He produced a tree of vertebrates, mollusks, plants and mammals, just to name a few.

Haeckel was making bold proclamations, but his work was really an extension of another man’s ideas: those of Charles Darwin.