Trees, not ladders
||Aristotle's vision of a Great Chain of Being, above. We now know
that this idea is incorrect.
Several times in the past, biologists have committed themselves to
the erroneous idea that life can be organized on a ladder of lower
to higher organisms. This idea lies at the heart of Aristotle's
Great Chain of Being (see right).
Similarly, it's easy to misinterpret phylogenies as implying that some
organisms are more "advanced" than others; however, phylogenies
don't imply this at all.
In this highly simplified phylogeny, a speciation event occurred resulting in two lineages.
One led to the mosses of today; the other led to the fern, pine, and rose. Since that
speciation event, both lineages have had an equal amount of time to evolve. So, although
mosses branch off early on the tree of life and share many features with the ancestor of
all land plants, living moss species are not ancestral to other land plants. Nor are they
more primitive. Mosses are the cousins of other land plants.
So when reading a phylogeny, it is important to keep three things in mind:
- Evolution produces a pattern of relationships among lineages that is
tree-like, not ladder-like.
- Just because we tend to read phylogenies from left to right, there is
no correlation with level of "advancement."
- For any speciation event on a phylogeny, the choice of which
lineage goes to the right and which goes to the left is arbitrary. The following
phylogenies are equivalent:
Biologists often put the clade they are most interested in (whether that is bats,
bedbugs, or bacteria) on the right side of the phylogeny.
Misconceptions about humans
The points described above cause the most problems
when it comes to human evolution. The phylogeny of living species
most closely related to us looks like this:
It is important to remember that:
- Humans did not evolve from chimpanzees. Humans and chimpanzees are evolutionary
cousins and share a recent common ancestor that was neither chimpanzee nor human.
- Humans are not "higher" or "more evolved" than other
living lineages. Since our lineages split, humans and chimpanzees have each evolved
traits unique to their own lineages.