Chopping down the tree of life

This brings us to the "tree of life." We've all seen "trees of life"; they begin with an original, pulsating, primordial slime-blob that eventually evolves into everything else as you trace the branches outward (Figure 3.3 is an example). Such trees show who came from whom, and when that occurred. They are common in textbooks and museum displays, and deeply influence our ideas about evolution. But how do we know who gave rise to whom? After all, no human witnessed the zillions of speciation events that constitute the "tree of life."

Pterosaur

Mammal (Bat)

Bird

Mammal (Bat)

Human

Figure 3.1. Homologs. Homologs are anatomical structures that can at least theoretically be traced back to a single structure in a common ancestor. The front limbs of humans, bats, birds, and pterosaurs are all homologous, and retain the same basic structure and bone relationships even though the appearance of these forelimbs may be outwardly different. Homology forms the basis for hypotheses of evolutionary relationships.

The problem is worse than that. We learned in Chapter 1 that fossil preservation is a rare event indeed. Is it likely that some fossil that we find is the actual ancestor of some other? The chances of this occurring are vanishingly small. Thus the oldest fossil in the human family is very unlikely to be the great great great granddaddy of all subsequent humanity.

Figure 3.2. Analog. Analogs may perform similar functions, and may even look outwardly similar, but internally they can be very different. Here a human leg is contrasted with that of a grasshopper. Although both have legs, the two structures are different. For starters, human muscles are on the outside ofthe skeleton, whereas grasshopppers' muscles are on the inside of their skeleton!

Figure 3.2. Analog. Analogs may perform similar functions, and may even look outwardly similar, but internally they can be very different. Here a human leg is contrasted with that of a grasshopper. Although both have legs, the two structures are different. For starters, human muscles are on the outside ofthe skeleton, whereas grasshopppers' muscles are on the inside of their skeleton!

But that fossil is likely to have many features that the real great great great granddaddy possessed. And therein lies the key to recognizing who is related to whom: while we'll never find the actual ancestor, we have a really good chance of finding out more or less how that ancestor looked. Because trees of life specify actual ancestors, and because, as we have seen, we're not likely to actually have the real ancestor in hand, we'll avoid trees of life, and instead use a revolutionary method to understand who is related to whom or, thinking of it another way, the course of evolution. And that method is called phylogenetic systematics.

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