Amoebas are puny, stupid blobs, so scientists were surprised to learn that they contain 200 times more DNA than Einstein did. Because amoebas are made of just one cell, researchers assumed they would be simpler than humans genetically. Amoebas date back farther in time than humans, and simplicity is considered an attribute of primitive beings.
It just didn’t make sense.
The idea of directionality in nature, a gradient from simple to complex, began with the Greeks, who called nature physis, meaning growth. That idea subtly extended from changes over an organism’s lifetime, to changes over evolutionary time after Charles Darwin argued that all animals descend from a single common ancestor. When his contemporaries drew evolutionary trees of life, they assumed increasing complexity. Worms originated early in animal evolution.
Creatures with more complex structures originated later. Biologists tweaked evolutionary trees over the following century, but generally, simple organisms continued to precede the complex.
Take the textbook scenario on early animal evolution. It essentially goes as follows: Single-celled organisms gained the ability to adhere to and communicate with one another more than 600 million years ago, and from the resulting colonies, the first multicellular animals emerged.
Today’s sponges, sedentary animals on the sea floor with no guts, brains, or tissue layers, descend directly from some of these creatures. Some early animals then organized their cells into distinct tissue layers, and some of the cells formed nerve cells, muscle cells, and other types.
Later yet, some animals developed serially repeated segments that served as a platform for legs and claws in their descendants. Then an animal with a spinal column evolved, and then one with a column surrounded by bony vertebrae. A recent branch to split from the tree blossomed into humans.