“I wonder if violence is innate in all intelligent species,” I said. “Evolution is driven by struggles for dominance. I’ve heard it suggested that no herbivore could ever develop intelligence because it doesn’t take any cunning to sneak up on a leaf.”

“It does create an odd dynamic,” said Hollus. “Violence is required for intelligence, intelligence gives rise to the ability to destroy one’s species, and only through intelligence can one overcome the violence that gave rise to that intelligence.”

“We’d call that a Catch-22,” I said. “Maybe we create the idea of a caring God and morality to foster self-preservation. Perhaps any race that doesn’t have morality, that doesn’t suppress its violent urges in a desire to please a god, is doomed to destroy itself once it gets the technology to do so.”

“An interesting thought,” said Hollus. “Belief in God conferring a survival advantage. Evolution would then select for it.”

“Does your race still worry about destroying itself?” I asked.

Hollus bobbed, but I think it was a gesture of negation, not affirmation. “We have a unified planetary government, and much tolerance for diversity. We have eliminated hunger and want. There is little reason for us to come into conflict with each other anymore.”

“I wish I could say the same thing about my world,” I said. “Since this planet was fortunate enough to have life arise on it, it would be a shame to see it snuffed out because of our own stupidity.”

“Life did not arise here,” said Hollus.

“What?” I was completely lost.

“I do not believe that there was a biogenerative event in Earth’s past; I do not believe life began here.”

“You mean it drifted here from deep space? Fred Hoyle’s panspermia hypothesis?”

“Possibly. But I suspect it is more likely that it began relatively locally, on Sol IV.”

“Sol — you mean Mars?”

“Yes.”

“How would it get here from there?”

“On meteors.”

I frowned. “Well, there’ve been a couple of Martian meteorites found over the years that some said had fossils in them. But they’ve been pretty thoroughly discredited.”

“It would only take one.”

“I suppose. But why don’t you think life is native to this planet?”

“You said you thought life had emerged on this world as much as four billion years ago. But that early in your solar system’s history, this planet was still routinely undergoing extinction-level impact events, as large comets and asteroids frequently slammed into it. It is extremely unlikely that conditions suitable for life could have been maintained during that period.”

“Well, Mars is no older than Earth, and surely it was undergoing bombardment, too.”

“Oh, doubtless so,” said Hollus. “But although Mars clearly had running water in its past — its surface today is really quite impressive to stand upon; the erosion features are incredible — it never had large or deep oceans like those here on Earth. If an asteroid hits land, heat from the impact might raise temperatures for a matter of months. But if it hits water, which, after all, covers most of Earth’s surface now as well as billions of years ago, the heat would be retained, raising the planet’s temperature for decades or even centuries. Mars would have had a stable environment for the development of life perhaps as much as half a billion years before Earth did.”

“And then some of it was transferred here, on meteors?”

“Exactly. About one thirty-sixth of all the material that gets knocked off Mars by meteor impacts should eventually be swept up by Earth, and many forms of microbes can survive freezing. It neatly explains why full-fledged life is recorded in the oldest rocks here, even though the environment was too volatile for it to develop domestically.”

“Wow,” I said, well aware that my response wasn’t adequate. “I suppose one meteor with life on it might have made it here. After all, every lifeform on this planet shares a single common ancestor.”

Hollus sounded astonished. “All life on this planet shares one common ancestor?”

“Of course.”

“How do you know that?”

“We compare the genetic material of different lifeforms, and, judging by how much it diverges, we can tell how long ago they had an ancestor in common. For instance, you’ve seen Old George, the stuffed chimpanzee we have in the Budongo Rain Forest diorama?”

“Yes.”

“Well, humans and chimps differ genetically by only 1.4 percent.”

“If you will forgive me for saying so, it does not seem right to stuff and display so close a relative.”

“We don’t do that anymore,” I said. “That mount is more than eighty years old.” I decided not to mention the stuffed Australian aborigine they used to have on display at the American Museum of Natural History. “In fact, it’s largely through genetic studies that the concept of ape rights gained credence.”

“And such studies show all life on this planet to have a common ancestor?”

“Of course.”

“Incredible. On both Beta Hydri and Delta Pavonis, we believe there were multiple biogenerative events. Life on my world, for instance, arose at least six times during an initial 300-million-year period.” He paused. “What is the highest level in your hierarchical biological classification system?”

“Kingdom,” I said. “We generally recognize five: Animalia, Plantae, Fungi, Monera, and Protista.”

“Animalia are the animals? And Plantae the plants?”

“Yes.”

“All animals are grouped together? Likewise all plants?”

“Yes.”

“Fascinating.” His spherical torso bobbed deeply. “On my world, we have a level above that, consisting of the six — well, ‘domains’ might be an appropriate translation — the six domains from the six separate creation events; separate kinds of animals and plants exist in each. For instance, our pentapeds and octopeds are, in fact, completely unrelated; cladistic studies have demonstrated that they share no common ancestor.”

“Really? Still, you should be able to use the DNA technique I described to determine evolutionary relationships amongst members of the same domain.”

“The domains have commingled over the eons,” Hollus said. “The genome of my own species contains genetic material from all six domains.”

“How is that possible? As you said about Spock, the idea of members of different species — even from the same domain — having offspring is ludicrous.”

“We believe viruses played a substantial role over millions of years in moving genetic material across domain boundaries.”

I thought about that. It had been suggested on Earth that unnecessary material transferred into lifeforms by viruses accounted for much of the junk DNA — the ninety percent of the human genome that did not code for protein synthesis. And, of course, geneticists today were deliberately transferring cow genes into potatoes and so on.

“All six domains are based on DNA?” I asked.

“As I have said, every complex lifeform that we have discovered is based on DNA,” said Hollus. “But with DNA crossing domains throughout our history, the kind of comparative study you suggest is not something we have had much success with. Animals that are clearly very closely related, based on the gross details of body form, may have significant recent intrusions of new DNA from another domain, which would make the percentage of deviation between the two species deceptively large.”

“Interesting,” I said. A thought occurred to me, too crazy to voice out loud. If, as Hollus said, DNA was universally used in all lifeforms, and the genetic code was the same everywhere, and lifeforms even from different domains could incorporate each other’s DNA, then why couldn’t lifeforms from different worlds do the same thing?

Maybe Spock wasn’t so improbable after all.