Did living things come from a common ancestor, or were they created separately?

The title asks an important question. It's also a broad question. Let me re-phrase it, more narrowly:

Do some living things resemble each other because of descent from a common ancestor, or because they were created to be similar?

Some people do not believe that the earth is old enough for there to have been distant common ancestors. They might believe that, say, cows and bison came from a common ancestor, or at least that all the breeds of cows did, but that cows and deer could not have, because there wasn't enough time. Some people, often the same ones, believe that the language of Genesis 1 indicates that God created the varieties of organisms by separate creative acts. Some people, usually the same ones as mentioned above, do not believe that humans and non-human primates share any common ancestry, because they believe that the language of Genesis 1 indicates that humans were created specially from non-living material. On the other hand, there are Bible-believing, God-fearing Christians, who are not so sure of all of this, and believe that the earth is very old. Such people may also believe that God especially endowed some pre-existing primate with the image of God (whatever that is) and that, therefore, non-human primates were distant biological ancestors of humans. Billy Graham has said that that may have been what happened.

If humans and chimpanzees were created separately, then how do people who believe this explain the similarities between the two? In particular, how do they explain the genetic similarity? The usual response is that, in order to function properly, genes have to fall within a narrow range of structural constraints. Therefore, human genes, and chimpanzee genes, were created to be much alike. Although we aren't really sure that there is only one way to make, say, an Oxygen-carrying molecule like hemoglobin, this argument of those who believe in special creation has some plausibility. But not enough.

A recent article, by a scientist who is a Christian, and well-qualified to discuss the matter, examines a situation where genes don't function. And, in this case, vitamin C production, it turns out that non-functioning genes of humans and non-human primates are very similar. The only reasonable explanation seems to be that humans and non-humans inherited their genes for vitamin C production from an ancestor which had lost the capacity to produce enzymes that could synthesize vitamin C. Thus neither humans nor orangutans can produce vitamin C, but both have the apparent remains of the genes that produced it in the presumed common ancestor, and these remains are almost identical.

Thanks for reading.

An argument (theological, actually) against Intelligent Design

keiths has recently posted an article challenging Intelligent Design. In particular it challenges the idea that the similarities between organisms are not the result of evolution by natural selection, and common descent, but due to the use of a common design, or pattern, for several organisms, by a Designer.

keiths's challenge is basically this: Why should a Designer have been limited only to designs that mimic exactly the sorts of results we would expect if similarity were due to evolution from common ancestor(s) by natural selection? In other words, why should a Designer be a mimic of evolution, when there must have been many other ways to Design living things?

This is, at least in part, a theological argument, although keiths doesn't say so. (I have no knowledge of his personal beliefs, except that he is clearly not a fan of Intelligent Design or Young-Earth Creationism.) Why should God (assuming God to be the Designer, which most proponents of Intelligent Design do) have been constrained in this way, and, also, why should God have mimicked the results expected from common descent through natural selection? Why would God have done this, when doing so presented abundant evidence for descent from common ancestor(s) by natural selection, if such common descent didn't actually occur?

Good questions, I believe.

Thanks for reading. Read keiths's post, if you wish.

Similarities in organisms: designed or evolved?

Different organisms have similar properties, including appearance, and less obvious similarities, such as similarities between their DNA and proteins. The mainstream scientific explanation for such similarities is that the organisms are related by descent. Those who doubt that organisms which belong to different large taxonomic groupings, such as families -- humans and chimpanzees belong to the family Hominidae, but gibbons to the family Hylobatidae -- could have descended from a common ancestor argue that the similarities are there because the chemicals were designed to perform similar functions.

A recent post on the Panda's Thumb blog points out some weaknesses in the design explanation.

The post points out that marsupial organisms have biochemical similarities which seem to be based on common descent and relationship, rather than on their ecological niche. Thus, marsupial predators have molecules that resemble other marsupials far more than they resemble those of other predators.

The post also points out that non-functional, or "broken" genes show similarities which indicate common descent. Since they don't function, it's hard to believe that their resemblances are due to design to carry out a similar function.

Thanks for reading.