Re: If science has shown that we evolve from the apes, why are we so different?

Thanks for your question Robert; I can tell that you've been thinking about the issues of human evolution for a while, so I will try and give you as thorough an answer as I can.

First of all, lets address one common misconception about the evolution of our species. Humans didn't actually evolve from apes. Apes are modern organisms, just as we are. Modern humans and the other so-called great ape species evolved from a common "ape-like" ancestor. This is reflected by the use of the word Hominoid to include all of these species (and their ancestors) in one group. When we call that ancestral species ape-like, we mean that this ancestor had certain characteristics that are seen in each of the modern great ape species. This also means that the common ancestor of apes and humans was "human-like," as we have inherited certain characteristics from that same common ancestor. However, you never hear anyone saying that apes are evolved from humans.

Now, as to the reasoning and evidence in support of the recent evolution of humans and the other great apes from a common ancestor, you have to look at the fossil record of human evolution and the molecular data derived from studies of human and ape DNA, as well as the DNA of other primates. The take-home messsage from all of this data is that human evolution and the evolution of the other great apes branched off from each other between 4 and 6 million years ago, but lets take a closer look at some of the recent discoveries that have been made in this field.

If you look at all of the other mammals, it is clear that humans are most similar to other primates, that we are in fact funny-looking apes. For a long time, people thought that it must have been at least twenty million years since our species shared a common ancestor with any of the other ape species, because it was thought that humans were extremely different from the other primates, and because we didn't really have many fossils from the ancestors of modern apes. However, in the late '70s, early genetic studies of humans and the other apes showed that there were many genetic similarities between humans and apes, and especially between humans, chimpanzees and gorillas. In some ways, it is impossible to tell a human from a chimpanzee or a gorilla.

Today, we know that humans, gorillas and chimpanzees are all descended from a common ape/human-like ancestor that lived about 4 - 6 million years ago because of what is called a "molecular clock." Most genes will accumulate changes (mutations) randomly, but at a regular, steady rate. This means that if two organisms are descended from a common ancestor, that the degree of difference between the genes that they share will be proportional to the amount of time that has passed since they shared a common ancestor. By comparing the genes that were common to many different primates (not just apes, but also so-called old-world and new-world monkeys), it was possible to determine the degree to which each species differed from the others. Then, by comparing the degree of difference between old-world and new-world primates (which is proportional to the time since those species shared a common ancestor) to the amount of time that had passed since South America and Africa were split apart (40 million years ago) and these two groups were permanently seperated, it was possible to calibrate the molecular clock (i.e. assign specific time-points to degrees of genetic difference), and arrive at an age of 4-6 million years for the common ancestor of humans, gorillas and chimpanzees.

Similarly, the molecular clock has been used to estimate the amount of time that has passed since the time of the common ancestor of all living people. Everyone alive today is descended from a common ancestor that lived approximately 130,000 - 150, 000 years ago. In addition, we know that this common ancestor lived in Africa, because the populations with the greatest genetic diversity are in Africa. Very recently, DNA from two different Neandertal bones was obtained and compared to DNA from modern people. As a result of this comparison, we know that Neandertals and modern humans shared a common ancestor about 500,000 years ago, and we think that it means that the Neandertal people were a separate species from our modern human species.

Finally, you asked about the specific details that make each of us distinct from any other modern human. It might seem to be the case that all chimpanzees or all gorillas are much more similar to other members of their own species than any of us humans are, but that really isn't true; its just hard for us to notice the differences. In fact, there are two separate species of chimpanzee. Pan troglodytes is the animal that we know as the chimp, and Pan paniscus is the animal called the Bonobo, or Pygmy Chimp. These two chimpanzee species shared a common ancestor that lived more than 2 million years ago, and if you look at any population of Chimps or Bonobos, you will find that their genetic diversity is larger than that for the entire human population ; even though our world-wide population vastly outnumbers theirs, we are all much more closely related to each other than most chimpanzees are.

I know that this has only been a brief answer, but I hope it helps you with your question. Please feel free to email me if you need further clarification.

The Fossil Record

There is so much fossil data that it is difficult to know where to start sometimes, but try taking a look at the Institute for Human Origins site, and also at these descriptions of fossil species and fossil specimens.

The Molecular Record

Here is a sampling of relevant papers and reviews from the last ten years.

Jin L, Underhill PA, Doctor V, Davis RW, Shen P, Cavalli-Sforza LL, Oefner PJ. (1999) Distribution of haplotypes from a chromosome 21 region distinguishes multiple prehistoric human migrations. Proc Natl Acad Sci U S A 7:3796-800

Quintana-Murci L, Semino O, Bandelt HJ, Passarino G, McElreavey K, Santachiara-Benerecetti AS. (1999) Genetic evidence of an early exit of Homo sapiens sapiens from Africa through eastern Africa. Nature Genetics 4:437-41

Su B, Xiao J, Underhill P, Deka R, Zhang W, Akey J, Huang W, Shen D, Lu D, Luo J, Chu J, Tan J, Shen P, Davis R, Cavalli-Sforza L, Chakraborty R, Xiong M, Du R, Oefner P, Chen Z, Jin L. (1999) Y-Chromosome evidence for a northward migration of modern humans into Eastern Asia during the last Ice Age. American Journal of Human Genetics 6:1718-24

Cavalli-Sforza LL. (1998) The DNA revolution in population genetics. Trends in Genetics 2:60-5

Krings M, Stone A, Schmitz RW, Krainitzki H, Stoneking M, Paabo S. (1997) Neandertal DNA sequences and the origin of modern humans. Cell 1 :19-30

Stoneking M, Soodyall H. (1996) Human evolution and the mitochondrial genome. Current Opinion in Genetic Development 6:731-6.

Goldstein DB, Ruiz Linares A, Cavalli-Sforza LL, Feldman MW. (1995) Genetic absolute dating based on microsatellites and the origin of modern humans. Proc Natl Acad Sci U S A 15:6723-7

Vigilant L, Stoneking M, Harpending H, Hawkes K, Wilson AC. (1991) African populations and the evolution of human mitochondrial DNA. Science 5027:1503-7.

Cann RL, Stoneking M, Wilson AC. (1987) Mitochondrial DNA and human evolution. Nature 6099:31-6