In what ways are you related to the contents of your salad? Or to the ingredients of a slice of pepperoni pizza? Or whatever your next meal might be? Of course, consumption is an ecological relationship. Your body digests and absorbs the nutrients from your food, which provide energy for your metabolism and material components for your cells. But another fundamental kind of relationship is more cryptic, and in many ways more profound.
The answer comes from one of Charles Darwin's least appreciated revolutionary ideas. Darwin is, of course, duly famous for his discovery of the process of natural selection, which is among the most successful concepts in the history of science. Darwin also discovered the process of sexual selection, which he viewed as an independent mechanism of evolution. But Darwin was the first person ever to imply that all of life came from a single or a few common origins, and had diversified over time through speciation and extinction to become the richness of the biotic world we know today. Darwin referred to the history of this diversification as "the great Tree of Life," but today biologists refer to it as phylogeny. It may be Darwin's greatest empirical discovery.
Network science is an exploding field of study. Network analysis can be used to trace neural processes in the brain, uncover terrorist groups from cellphone metadata, or understand the social consequences of cigarette smoking and vaping among cliques of high school students. Biology is a network science. Ecology investigates the food web, while genetics explores the genealogy of variations in DNA sequences. But few understand that evolutionary biology is also a network science. Phylogeny is a rooted network in which the edges are lineages of organisms propagating over time and the vertices are speciation events. The root of the phylogenetic network is the origin of life as we know it—diagnosable by the existence of RNA/DNA-based genetic systems, left-handed amino acids, proteins, and sugars, and (likely) a lipid bilayer membrane. These are the features of the trunk of Darwin's great Tree of Life.
Thus, you are related to the lettuce, the anchovies, the Parmesan, and the chicken eggs in your Caesar salad through the historical network of shared common ancestry. Indeed, there is nothing you could think of as a food that cannot be placed on the Tree of Life. Being a member of this network is currently the most successful definition of life.
Darwin should be world famous for his discovery of phylogeny. But, just as Einstein's discovery of the quantum nature of energy was eclipsed biographically by his discovery of the theory of relativity, Darwin had the mishap of discovering natural selection too. Despite its excellent intellectual roots, phylogeny remains underappreciated today because it was largely suppressed and ignored for most of the 20th century. The architects of the "New Synthesis" in evolutionary biology were eager to pursue an ahistorical science analyzing the sorting of genetic variations in populations. This required shelving the question of phylogeny for some decades. As population genetics became more successful, phylogeny came to be viewed merely as the residuum left behind by adaptive process. Phylogeny became uninteresting, not even worth knowing.
But the concept of phylogeny has come roaring back in recent decades. Today, discovering the full details of the phylogenetic relationships among the tens of millions of extant species and their myriad of extinct relatives is a major goal of evolutionary biology. Just like a basketball tournament with sixty-four teams has sixty-three games, the phylogeny of tens of millions of living species must have tens of millions minus one branches. So, biologists have a lot of work ahead. Luckily genomic tools, computing power, and conceptual advances make our estimate of organismal phylogenies better and more confident all the time.
Despite a lot of empirical progress, the full implications of the concept of phylogeny have yet to been appreciated in evolutionary biology and the culture at large. For example, the concept of homology—similarity relation among organisms and their parts due to common ancestry—can only be understood in terms of phylogeny. Infectious diseases are caused by various species from different branches on the Tree of Life. Defending against them requires understanding how to slow them down without hurting ourselves, which is greatly facilitated by understanding where they and we fit in the historical network of phylogeny.
Billions of dollars of biomedical research funds are invested into a few model organisms like E. coli, yeast, round worms, fruit flies, and mice. But, like the diverse contents of your salad, these scientific results are usually not consumed with any awareness of the complex hierarchical implications of the phylogenetic context.
Perhaps the most important implication of the singular phylogenetic history of life is its contingency. Given the pervasiveness of extinction in pruning the network, our existence, or the existence of any other extant species, is only possible as a result of an unfathomable number of historically contingent events—speciation events, evolutionary changes within lineages, and survival. The history of any one branch connects to the whole, individualized history of life.