Re: what is the rate of evolution in humans and does it vary?

My first thought as I read your question was "Good question!". This is a code phrase scientists use because it sounds better than saying "I don't have any idea." or "Please don't open that can of worms.".

Actually, we do know a lot about the rate of mutation and the rate of evolution of primates and other mammals. But the first thing to understand is that these rates are probably not exactly what you and most other people are interested in. For example, the fossil record for hominids (modern humans, neanderthals, chimpanzees, gorillas and other "great apes") is not "complete" but it is pretty well filled in compared to the fossil records for most other mammals, because humans are always most interested in themselves and their relatives. So we know from the fossil record that humans and chimpanzees split from their most recent common ancestor between 5 and 12 million years ago. We also know from sequencing the genes of chimpanzees, humans, gorillas and other primates that the DNA sequences of humans and chimpanzees are between 98.4% and 99.6% identical even after all those years. So the rate of mutation of great ape germ-line DNA is roughly 2.6% per 8 million years or 0.35% per million years or 3.5 mutations per 1 billion bases (the human genome contains 3 billion bases) of DNA each year. In theory, most of primate DNA is "junk DNA" that does not code for any proteins and mutations in all that DNA have no effect, either positive or negative. It is only mutations in genes that should change the behavior or "phenotype" of the individual. (The phenotype is anything we can see or observe. The genotype can only be detected by examining the DNA, such as by sequencing or by RFLP analysis.)

But that sort of information tells us next to nothing about the really exciting things that have happened to make humans different from chimpanzees or chimpanzees different from Gorillas. Some of the more exciting things are events that cause speciation. One example is a change in chromosome number. All the great apes, that is both major taxa of Pan, P. paniscus (bonobo) and P. troglodytes (‘common’ chimpanzee) as well as the subspecies of Gorilla and the subspecies of Pongo (orang- utan) have 48 chromosomes whereas humans have 46. This is actually just a fusion of two chromosome into one, because there are 23 pairs of chromosmes to make the 46 and 24 pairs to make the 48. While the mutations and evolution rates discussed above accumulate very slowly and gradually over time, changes in chromosome number, or major rearrangements of chromosmes (an arm of one chromosome trading places with an arm of another) are sudden major changes that can cause speciation. These major changes in chromosomes, althought they prooduce almost no change in any one gene sequence, can also cause major changes in the phenotype. One example is how people with Down Syndrome, which is a duplication of chromosome 21, have many features in common.

The point is, that there is great potential for rather minor changes in DNA to make major changes in the phenotype. This is one of the mechanisms that can explain what the fossil hunters observe as "jumps" in evolution. The right rearrangement can cause a "jump" or major change in the phenotype, while being a rather minor change in DNA sequences.

In addition to chromosomal rearrangements, there are also many "jumping genes" or transposons and endogenous retroviral elements that can have major impacts on phenotypes. Barbara McClintock won a Nobel prize for her work in studying jumping genes in corn, which she began researching before DNA sequencing technology became available. An essay about this famous woman scientist is found in the Brown University Book Review of a book about her.

The end result is that while DNA mutations happen fairly regularly, there are all sorts of twists in the plot that make the end result of observable phenotype changes happen in very interesting jumps and turns.

I found a long and detailed essay on primate chromosome number here.