of the story. Now I wanted his.
“I was told on Goodfellow that DNA-based life has been found in the outer system of Earth’s star—micro-organisms deep-frozen for billions of years,” I said, broaching the matter as forthrightly as I dared, without running the risk of offending him. “The Tetrax must have had a chance to study thousands of life-bearing solar systems. How many are like ours in this respect?”
“Nearly all of them,” he said, lightly. “I know of one or two anomalous cases, but we have concentrated our researches on stars of the same solar type, whose planetary systems are roughly similar.”
“That seems to indicate that life didn’t evolve in any one of them—in fact, that there’s no way of knowing where DNA first came from.”
“We certainly have no basis for speculations about the ultimate origin of life,” admitted the Tetron.
“My ancestors always supposed that life evolved on Earth,” I said, carefully angling for more information. “Even when we came out into space and found the other humanoid races, we clung to that idea, and invented theories of convergent evolution to save it.”
“Our scientists never supposed that to be the case,” he informed me, with a touch of that lofty superiority that the Tetrax love to display. The best way get them to tell you something is to play up to that vanity.
“How did they work that out?” I asked, trying to sound suitably awed.
“A simple matter of the elementary mathematics of probability. The basic chemical apparatus of life is very complex. It is not only DNA itself, but all the enzymes associated with it, and the various types of RNA involved in transcription of the genetic code. It was easy to work out the probability of such a system arising by the random accretion of molecules. When we compared that probability to the area of our planet and the length of time since its origin, it was perfectly obvious that the chance of life originating there—or on any other planet—was absurdly small.
“It was obvious to us that the chemistry of life is so complicated that its evolution by chance would require vast areas of space and incredible spans of time. Our best estimate is that, given the size of our universe, the length of time for which we expect it to endure, and the kind of life-history we expect it to follow, the odds against life evolving at all were about ten to one against. It would appear that we owe our existence to a remarkable stroke of luck.”
I didn’t ask him to explain the mathematics of this remarkable calculation, but I took it with a pinch of salt. The trouble with the calculus of probability is that you can easily get silly answers if there are factors operating which you don’t know about. Ludicrous improbabilities are ten a penny in scientific research.
“Does that explain why the life-systems of the homeworlds of all the galactic races are so very similar?” I asked.
“Not in itself,” he told me. “If your world and mine had simply received the same elementary biochemical system, in the form of bacteria and virus-like entities, natural selection might have built very different systems. The fact that the pattern is repeated so closely, to the point where the insects of Tetra are very similar in their range to the insects of Earth—and so on for all the other major groups—implies that each of our worlds was seeded more than once. We think that new genetic material drifts from the outer to the inner regions of solar systems more-or-less constantly, and that this provides a major source of variations upon which natural selection can work; but we also think that seedings of more complicated genetic packages have occurred two or three times in recent galactic history—within the last billion years, that is.”
“So you think that the humanoid gene-complex was actually dumped on the inhabited worlds we know—by godlike aliens using the whole galactic arm as a kind of garden?”
Tetrax can’t frown, but I could tell that he thought I was going way over the top, and he clearly didn’t want such implications read into his argument. “We could not isolate the humanoid gene-complex as such,” he said. “At present, our best theory is that the last seeding may have been done at the time when, in Earthly terms, the dinosaurs died out. That radical break in the evolutionary story is something which recurs on many worlds, but there is no reason to suppose that alien intelligences were responsible for the seeding.”
“But you are saying that the mammalian gene-complex came from outer space, not from the DNA that already existed on Earth or Tetra?”
“That seems to be the case,” he confirmed. He looked at me carefully for a minute or two, perhaps wondering how much I would be able to understand. I got the feeling that we were now getting close to his own hobby-horse. “Do you know what is meant by the phrase ‘quiet DNA’?” he asked.
“No,” I replied. I began to suspect that we mightn’t get much further. Pan-galactic parole is a language designed to be easy to use. It isn’t geared up for complicated scientific discourse, and my limited mastery of it might soon come up against its limitations.
“Your gene-mappers, like ours of a few centuries ago, have now succeeded in locating on mammalian chromosomes—including human chromosomes—the genes that produce all the proteins making up your bodies.”
He paused, and I said: “Okay—I understand that.”
“Those genes,” he said, “account for somewhere between five and ten percent of the DNA in your cells. The rest is ‘quiet DNA’.”
“What you mean,” I said, in order to demonstrate my intelligence, “is that nobody knows what it does.”
“Quite so. Our scientists thought for some time that it must be made up of genes to control other genes. You see, there is more to building an organism than a mere chemical factory. An egg-cell, as it develops into a whole organism, must not only produce the proteins it needs, but must organize them into a particular structure. For many years our biotechnologists have tried to discover how it is that an egg is programmed to develop into a particular kind of organism. We had always assumed that the answer lay in the quiet DNA. We have failed to solve the problem. Your own biotechnologists are just beginning to be frustrated by that barrier to progress. We have found many practical applications for our biotechnology, and have been able to accomplish many things in spite of our incomplete understanding, but we must reluctantly acknowledge that one of the basic features of the chemistry of reproduction is still a complete mystery.
“What we have discovered, however, is that the quiet DNA of many—perhaps all—lower mammals includes genes which are expressed only in higher forms.”
I was having a little difficulty in following this, and had to pause for thought, but I suddenly saw what he was getting at. “You mean,” I said, “that virtually all the genes coding for the bodies of humanoids were already in mammals when they first appeared on Earth—or Tetra—and that the subsequent evolution of the mammals has been partly a matter of that quiet DNA waking up.”
He looked a little surprised.
“That’s correct, Star-Captain Rousseau,” he said. “In my view, at least, that is a distinct possibility—although it remains as yet unproven. The evolution of mammalian forms is, we think, partly pre-programmed. The program has to be adapted by natural selection to fit local circumstances, but in essence, the evolution of intelligent humanoid life-forms on all the worlds of the galactic community was inevitable from the moment the mammalian gene-complex appeared there. The subsequent millions of years of evolution can be seen as a kind of unfolding of potential already contained in the DNA-complex.”
I found
