George Wald: The Origin of Death

George Wald: The Origin of Death

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This was perhaps my father's greatest scientific talk, a perfect balance of genuinely exploratory scientific thought and a popular lecturing style that had earned him a place in Time magazine's cover story on the twenty greatest teachers in America. I wish I could include the slides he used to show, which ranged from scientific charts to cartoons from the New Yorker.

The Origin of Death<br>&copy; 1970 George Wald

When one has wondered over the years about the origin<br>of life, as I have done, one comes inevitably to ask oneself, just what<br>kind of thing is one trying to bring forth? Need those first primitive<br>organisms on the earth, for example, have had such complex apparatuses<br>of reproduction as all organisms possess today? And then one comes to<br>the curious question: Need those first organisms have died? Because if<br>they didn’t need to die, they needn’t at least be in such a<br>hurry to reproduce. And this brings one to this question of the origin<br>of death.

For not all living creatures die. An amoeba, for example,<br>need never die; it need not even, like certain generals, fade away. It<br>just divides and becomes two new amoebas.

In fact, death seems to have been a rather late invention<br>in evolution. One can go a long way in evolution before encountering an<br>authentic corpse. This is the journey that I would like to make with you.<br>What I should like to do, of course, is to begin with the first living<br>organism on this planet and then pursue evolution onward, asking the question:<br>When did the first organism appear that cultivated the habit of dying?<br>But that is just what I can’t do. As in so many other evolution stories,<br>I have to be content with a poorer thing, and that is to discuss this<br>transition in terms of contemporary organisms, of organisms alive today.

Let us begin with a familiar, single-celled organism,<br>the amoeba. Its nucleus divides by pinching into two equal halves, then<br>the whole amoeba divides. Thus we have two organisms where we started<br>with one. This is the usual way singlecelled organisms, plant and animal,<br>tend to reproduce, just by simple division: so called fission.

Occasionally they do something a little different. Reproduction<br>in the single-celled organism Paramecium is usually by fission, but sometimes<br>it engages in what we call conjugation. Two organisms, each containing<br>a large nucleus (macronucleus) and a small nucleus (micronucleus), come<br>together side to side. Then the cuticle breaks down between them. The<br>macronucleus is by and large the working nucleus. The micronucleus represents<br>a store of genetic material. The next thing that happens is that the macronuclei<br>disintegrate and the micronuclei divide, and something very interesting<br>happens that makes one think a little of sexual reproduction: there is<br>an exchange of micronuclei, of genetic material. Then the Paramecia separate,<br>the micronuclei divide repeatedly, then the Paramecium divides repeatedly.<br>One ends up with eight brand new Paramecia just like the pair with which<br>we started.

In the generation just before mine there was a very distinguished<br>zoologist named Lorande Woodruff. He began to publish a series of papers,<br>the first of which was entitled something like “Two hundred generations<br>of Paramecium aurelia without conjugation.” We waited a few years<br>and another paper came out with a title something like “Five hundred<br>generations of Paramecium aurelia without conjugation.” Finally this<br>series reached its culmination in a paper entitled “Eleven thousand<br>generations of Paramecium aurelia without conjugation.” So Professor<br>Woodruff lived a happy and useful life, and convinced all of us that Paramecium<br>can live indefinitely without conjugation.

But in the course of these researches Woodruff made another<br>discovery. You see, every morning he’d come into his laboratory and<br>find two Paramecia where he’d left one the night before; so he’d<br>carefully separate them. One Paramecium, he thought, can’t conjugate.<br>But that’s where he was fooled, because watching these Paramecia<br>so intently he discovered still a third wrinkle in this process which<br>he called endomixis. It’s a sort of do-it-yourself conjugation. In<br>endomixis the macronucleus in a Paramecium disintegrates, the micronucleus<br>divides, one of those new micronuclei grows up to a new macronucleus and<br>you have a brand new Paramecium.

Then there is a fourth process, which is very interesting.<br>It is called syngamy. In syngamy two cells fuse to make one; and that,<br>of course, is essentially what happens in sexual reproduction. So, here<br>we have, just among these single-celled organisms, four different ways<br>of going about reproduction but no necessary dying, no corpses.

Let us now take an enormous jump in evolution, to a lower<br>invertebrate, the sea anemone. We’ve gotten from a...