Studying DNA may reveal much about human origins |

Studying DNA may reveal much about human origins

A century-plus ago, some biologists claimed they could see, under the microscope, a miniature human being – a “homunculus” with arms, legs, fingers, toes and all the other parts in place – within the head of a sperm. All that was needed was for this miniature human being to start growing when implanted within mom’s belly. Later biologists hypothesized more sophisticated models, in which each of the tissues within our bodies was present, as a template, within the fertilized egg.

Neither sperm nor egg contain any such templates, much less a miniature human. Rather, both contain the instructions for putting a human together. The instructions are written in code, and carried within molecules of DNA.

The code is like an alphabet, with three-letter “words,” one after another, telling our cells how to construct themselves. Just as in a book, it is the sequence of letters that is all-important.

The DNA code is universal. The instructions for building every organism on Earth are written in virtually the exact same code (exceptions are minor). It is the universality of the code that allows us to, say, place the gene for human insulin into a bacterium, step back, and watch the bacterium produce the substance, a hormone it has never needed.

When a cell reproduces and its DNA is bequeathed to its “daughters,” the error rate in the transmission is typically less than one in a million (no record-keeping bureaucracy could ever hope to match that degree of accuracy).

But one in a million is not perfection. If it were, we would not have eyes and brains to read this, but would be single-celled creatures, identical to our ancestors of 4 billion years ago. It is the occasional copying mistake that provides the raw material for change, for evolution.

If the mistake is only minor, the resulting molecule will be only slightly different from the original, and might still be able to carry out its function (perhaps better than the original). As horses and pigs and chimps and people have evolved away from each other, our DNA has also slowly diverged. The differences – how many there are – in the sequences of “letters” in our DNA gives us a strong indication of how long it’s been since we’ve shared a common ancestor.

(It was such comparisons that allowed researchers to determine that the anthrax recently mailed around the country was made in the U.S. – the DNA sequences were identical to anthrax prepared by the military.)

Within 10 years or so, robot spacecraft might bring samples of Mars back to Earth, samples which will be searched for life.

Meteorites can travel between worlds and could, conceivably, carry living hitchhikers from one to the other. Bacteria on Mars, if they exist, could have come from Earth (or Earth’s earliest inhabitants could conceivably have come from Mars), and so could code their genes in DNA. In that case, could we ever be sure that microbes found in Mars rocks had actually lived on Mars, and had not, rather, been carried there by a contaminated space craft, and then brought back to Earth by that same craft?

The question should be fairly easy to answer by comparing DNA sequences. Even if Earthlings and Martians share a common ancestor, the planet-seeding crossing of the sea of space would likely have taken place so long ago that the sequences of Earthly and Martian microbes would be very, very, very different.

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