Roughly 5 billion unique life forms have appeared on the Earth in the past 3.8 billion years, and 99% are extinct. Roughly 10-14 million discrete species are alive today, though only 1.2 million have been catalogued. Evolutionary biology is the subfield that studies the processes that produced this remarkable diversity. Over the past several decades, new genetic tools have upended this field and radically altered the consensus view of evolutionary mechanics. It turns out the “tree of life” is not a purely linear system where genetic inheritance is derived solely from ancestral lineage. The truth is more magical.

Evolution theory began with Gregor Mendel’s research on the “factors” governing color and other characteristics in peas. This led him to infer the basic rules governing inheritance, specifically dominant vs. recessive traits. A century later DNA was revealed to be the molecule chiefly responsible for the transmission of Mendel’s factors. In evolution 1.0, the chromosomal fusion of two parental gametes determines the inheritance of the offspring.

Chromosomal heredity does not explain how novel traits arise. In his 1859 book On the Origin of Species, Charles Darwin provided an explanation. In what he called natural selection, a differential in survival and reproduction in response to environmental changes drives the emergence of new characteristics over time. When genetic mutations confer an advantage in survival or reproduction, they replace legacy genes by outcompeting.

In a parallel process, the prioritization of certain traits when selecting a mate also favors the amplification of some characteristics within a species, which Darwin called sexual selection. For example, peacocks evolved gaudy tail feathers because peahens tended to select the suitor with the most vibrant plumage to mate with.

Vertical evolution holds that mutation, environmental adaptation, and changes in sexual preferences are the primary agents driving the creation of new traits and species. The problem is, this model doesn’t account for the genomic lineage of many creatures, including some of the biggest leaps in cell evolution. Version 2.0 of evolution fills these gaps with two additional mechanisms: horizontal gene theory (HGT), articulated by biophysicist Carl Woese; and serial endosymbiosis theory (SET), proposed by evolutionary biologist Lynn Margulis.

Carl Woese pioneered genetic research that changed the taxonomy of biology by adding another trunk onto the tree of life. Woese created a technique that uses a ribosomal RNA gene to gauge the “relatedness” between organisms. This enabled scientists to look at different species and determine how long ago their genomes diverged.

Previously, tree of life maps classified early cellular life into just two categories, prokaryotes and eukaryotes. Prokaryotes are organisms lacking a cell nucleus, such as bacteria. Woese’s research proved that a primitive group of microbes - archaea - are actually a third Domain of early life, not a subset of bacteria as previously believed.

Two revelations emerged from Woese’s work. First, all plants, animals, and fungi are descended from archaea. Second, horizontal gene transfer is common. Also called “infective heredity,” HGT explains how genes jump between species. It cements the view that humans are composite entities. In fact, genes absorbed from retroviruses now constitute 8% of the human genome. One snippet of viral DNA, for example, was repurposed in humans to enable the creation of the placenta during pregnancy.

Lynn Margulis was one of the most controversial and influential scientists of the 20th century. In 1967 she published her first description of SET, titled On the Origin of Mitosing Cells. By invoking Darwin in her title, Margulis signaled her view that endosymbiosis is a primary mechanism of evolution. This assertion was widely scorned until the weight of supporting evidence became overwhelming. Today SET is part of biological consensus.

An endosymbiont is any organism that lives within the body or cells of another organism. Margulis argued that complex life could not have evolved without microbial symbionts. She and chemist James Lovelock expanded SET to a global model known as the Gaia Hypothesis, which argues living organisms interact with their inorganic surroundings to form a synergistic, self-regulating system that perpetuates the conditions for life. Research has identified a burgeoning list of endosymbionts, but mitochondria and chloroplasts are still the best examples of the profound role of symbiosis.

An organelle is a subunit that resides inside a cell but outside its nucleus. Mitochondria are organelles found in most of the cells of eukaryotic organisms. Their primary role is producing ATP, a chemical that stores energy like a battery, and regulating cellular metabolism. Large organisms could not exist without mitochondria. Margulis proved that mitochondria actually evolved from bacterial symbionts. Likewise, the chloroplasts that enable plant cells to create energy from photosynthesis were once free-living cyanobacteria that became symbionts.

As it turns out, life is more than just a competition to survive and mate. The history of life is a tale of mutualistic collaboration and gene sharing between strangers. The plant and animal cells that exist today originated from symbiosis, and cross-species gene transfer caused some of evolution’s greatest leaps. Life is a contact sport, and that endless microscopic scrum is actually part of the magic that drives innovation.