2.2: Taxonomy

The two classic lines of evidence have been comparative anatomy and fossils. Recently, genetics has almost completely taken over comparative anatomy research, because it's like going to the primary source, the gene is what codes for the anatomy. It used to be that using fossils was the only way to get dates, but recently, the differences between the mutations in the mitochondrial DNA (mtDNA) of a species can be used to calculate the time when that species split off from another species with different mtDNA.

The fossil evidence gives us a picture that looks like this:

Figure \(\PageIndex{1}\): "The Geologic Time Spiral: A Path to the Past" by Joseph Graham, William Newman, and John Stacy (USGS) 2008) (Public Domain)

The genetic evidence looks like this:

Figure \(\PageIndex{2}\) - Genealogy of life by genetic distance , "The Tree of Life" by David M. Hillis, Derrick Zwickl, and Robin Gutell, University of Texas © 2003

Both imply the differences between living forms and the time when they branched off from each other. The genetic evidence is more concrete about living forms, the fossil evidence is more concrete about extinct life forms and gives more accurate dates.

Taxonomy is another way to think of your family tree. Biologists use different methods to make taxonomies, such as cladistics (better science) and evolutionary systematics (includes dates of speciation events) but as an anthropologist, I tend to emphasize the "genealogy" of classification – how a particular life form fits into my family tree. I try to figure out what part of my DNA I share with that organism, and probably inherited from a common ancestor. With mitochondrial DNA dating, we can get pretty close to calculating when the common ancestor between two life forms lived and when the split started.

For this class you can use most terminology interchangeably: taxon = clade = branch = phylum = group.

The whole family tree metaphor has recently come under attack by the evolutionary implications of processes like lateral gene transfer, and fuzzy hybrids, and we are learning more and more that evolution doesn't always follow a nice clean straight line. As we saw in the section on lateral gene transfer, there have been examples of two species interchanging DNA without having sex. Also, the concept of a species is not as rigid as we'd like to make it. Every once in awhile the normal sterile hybrid offspring of two different species can be fertile (Horse + Donkey = normally sterile mule).

These events are rare, so it's still worth learning classic evolutionary theory, but we can extrapolate from Charles Lyell's principle of Uniformitarianism that small changes can cause big effects over long periods of time. Call me stubborn, but I'm still not ready to give up on the family tree metaphor. LGT is good evidence that evolution doesn't follow the kind of clean branches that Darwin first sketched, but I think we can still go back to trees to save the metaphor and illustrate how life separates over time into distinct species, but can still occasionally come back together to share genetic material. Have you ever seen how when two separate roots or branches grow right next to each other for a long time that they sometimes fuse and become one root or one branch?

Exercise \(\PageIndex{1}\)

SKIM THIS PAGE OF TREES OF LIFE

The theme of separation and reconnection plays an important part in the interpretation of our own recent evolutionary history.

Figure \(\PageIndex{4}\) - Reed DL, Smith VS, Hammond SL, Rogers AR, et al. (2004) Genetic Analysis of Lice Supports Direct Contact between Modern and Archaic Humans. PLoS Biol 2(11) (CC BY 4.0)

Figure \(\PageIndex{5}\) - Human taxonomy, based on Stringer, C. (2012). "What makes a modern human". Nature 485 (7396): 33–35. (CC BY 4.0)

Figure \(\PageIndex{6}\) - Extension to 600 kya (Homo sapiens). (CC BY 4.0)

Note

Another good article about the "tree of life" metaphor

Astrobiology

This class focuses on the planet earth where humans evolved and share ancestry with all known life. But, it's fascinating to think of what life on other planets may be like. There are some serious hypotheses that some evolution may have taken place on other planets and been brought to the earth through meteors

Note

Life in extreme environments.