Posted by: Lister | February 18, 2007

Why we think that Humans and Chimps have a common ancestor

Note that the following isn’t meant to illustrate evolution. I’ve given the post a tighter remit because one of the main sticking point seems to be the common ancestry between humans and chimps.

Basically, I’ve made an analogy to the copying of jokes in order to illustrate how we might guess what has been copied from what. When scientists compare Human/Chimp DNA, they find it is very similar. They conclude it has been copied from a single source. And, since copying DNA involves making descendants, that common source was a common ancestor. I could stop there, but would that be fun?

Talk Origins is a great site for evolution queries in general. (And written by experts rather than amateur bloggers and hacks!) They’re also the source of my cytochrome c post on common descent.

What is the common answer to the age old conundrum: Why did the chicken cross the road? I got an e-mail with many answers.

Machiavelli’s answer ::: So that its subjects will view it with admiration, as a chicken which has the daring and courage to boldly cross the road, but also with fear, for whom among them has the strength to contend with such a paragon of avian virtue? In such a manner is the princely chicken’s dominion maintained.

Thomas de Torquemada ::: Give me ten minutes with the chicken and I’ll find out.

Douglas Adams ::: Forty-two.

Nietzsche ::: Because if you gaze too long across the Road, the Road gazes also across you.

B.F. Skinner ::: Because the external influences which had pervaded its sensorium from birth had caused it to develop in such a fashion that it would tend to cross roads — even while believing these actions to be of its own free will.

Albert Einstein ::: Whether the chicken crossed the road or the road crossed the chicken depends upon your frame of reference.

Buddha ::: If you meet the chicken on the road, kill it.

Salvador Dali ::: Three bangs of light and a hippopotamus.

Ernest Hemingway ::: To die. In the rain.

Henry David Thoreau ::: To live deliberately … and suck all the marrow out of life.

Zeno of Elea ::: To prove it could never reach the other side.

The truth ::: It was too far to walk around!

A quick google search will reveal: Philosophy E-Server has a list of answers very similar to those I got by e-mail.

Even though there are differences between the e-mail I received and the link google found, I think it is obvious that the two have a common source.

What that source may be, I don’t know. But there are too many similarities for it to be coincidence. I would guess that the list has been getting longer and longer as it passed from person to person — with some changes added here and there. EG: the link has a different quote from Buddha.

Buddha ::: If you ask this question, you deny your own chicken-nature.

And there’s another Buddha quote at Extremely Smart (He had a lot to say on the matter, obviously!)

Buddha ::: “Therefore, on the road there is no chicken, no road, nor perception of the road, nor impulse to cross it, nor consciousness of the road, no feathers, no beak, no clawed feet, no chicken. No road no chicken no crossing… only the great prajnaparamita of the empty form of chicken and the empty form of the road, and that emptiness; gone, gone, gone beyond, gone altogether beyond. ‘But, O Buddha,’ said Sariputta, ‘what is that crossing the road before us at this moment?’ And the Great One replied, ‘A chicken, Sariputta.’ ‘But why, O Great One, does it cross the road?’ ‘To get to the other side, Sariputta.’ Om.”

Dr. Seuss ::: “Did the chicken cross the road? Did she cross it with a toad? Yes! The chicken crossed the road, but why she crossed, I’ve not been told!”

Fox Mulder ::: “You saw it cross the road with your own eyes. How many more chickens have to cross before you believe it?”

Marcel Marceau ::: “ ”

That second link is almost totally different to the e-mail I received. Maybe people were inspired by the same source. But even so, they didn’t both copy that source.

And that’s how we know Chimps and Humans have a common ancestor.

I forgot to say.
Humans and Chimps share a lot of DNA. The odds that it wasn’t copied from a common source are extremely low. We also have DNA in common with all life on Earth — All life on Earth has a common source.

One of the things we share with Chimps is that we can almost make our own vitamin C. Most mammals can, we can’t. But we do carry the ghost of a gene that could…

Plagiarized Errors and Molecular Genetics

Guinea pigs and primates, including humans, get sick unless they consume ascorbic acid in their diet. For humans and guinea pigs, ascorbic acid is thus a vitamin (vitamin C), while most other species can synthesize their own ascorbic acid and thus do not require this molecule in their diet. The reason humans and guinea pigs cannot manufacture their own ascorbic acid is that they lack a functional gene encoding the enzyme protein known as L-gulono-gamma-lactone oxidase (GLO), which is required for synthesizing ascorbic acid. In most mammals functional GLO genes are present, inherited – according to the evolutionary hypothesis – from a functional GLO gene in a common ancestor of mammals. According to this view, GLO gene copies in the human and guinea pig lineages were inactivated by mutations. Presumably this occurred separately in guinea pig and primate ancestors whose natural diets were so rich in ascorbic acid that the absence of GLO enzyme activity was not a disadvantage — it did not cause selective pressure against the defective gene.

Molecular geneticists who examine DNA sequences from an evolutionary perspective know that large gene deletions are rare, so scientists expected that non-functional mutant GLO gene copies — known as “pseudogenes” — might still be present in primates and guinea pigs as relics of the functional ancestral gene. In contrast, Creationists believe that humans and guinea pigs were each created independently of all other species and must have been “designed” to function without GLO. If this were true, these two species would not be expected to carry a defective copy of the GLO gene. In fact, GLO pseudogenes have been detected in both guinea pigs and humans (Nishikimi et al. J Biol Chem 267: 21967, 1992; Nishikimi et al. J Biol Chem 269:13685, 1994), consistent with the evolutionary view; presumably, related pseudogenes also exist in non-human primates that require dietary vitamin C.

Note that the theory of evolution made a prediction. Scientists expected to find something before they saw it. The theory of evolution isn’t just description — it predicts.

Evidence for Macroevolution

We now have the DNA sequences for this broken gene [LGO] in chimpanzees, orangutans, and macaques (Ohta and Nishikimi 1999). And, as predicted, the malfunctioning human and chimpanzee pseudogenes are the most similar, followed by the human and orangutan genes, followed by the human and macaque genes, precisely as predicted by evolutionary theory. Furthermore, all of these genes have accumulated mutations at the exact rate predicted (the background rate of mutation for neutral DNA regions like pseudogenes) (Ohta and Nishikimi 1999).

The e-mail I received matches (very closely) the first link of chicken jokes — and I conclude there was some copying involved.

The malfunctioning LGO matches most closely between Humans and Chimps. In this case, copying implies descent from a common ancestor. And the relative closeness of the matches also tells us which of chimpanzees, orangutans, and macaques are more recently related to Humans.



  1. John Grehan posted on my “about” page regarding the theory that orangutans may be our closest relative.

    I’ve started a thread at the JREF forums to discuss the theory.

  2. When did humans cease to be able to manufacture vitimin c?

  3. Hi, Lee
    According to wiki, the relevant mutation happened about 63-58 million years ago. So it would be our non-Human ancestors who lost the ability.

    Mutations can be good, bad or indifferent. In the case of our ancestors, when the mutation happened they already had a diet which was rich in vitamin C. So the loss of the ability to make it themselves wasn’t too big a burden.

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