“Yes,” said Hollus. “Continue.”
“Well, and the most-famous mass extinction happened sixty-five million years ago, at the end of the Cretaceous.” I indicated the Troodon skull again. “That’s when all the dinosaurs, pterosaurs, mosasaurs, ammonites, and others died out.”
“This creature would have been rather small,” said Hollus, hefting the skull.
“True. From snout to tip of tail, no more than five feet. A meter and a half.”
“Did it have larger relatives?”
“Oh, yes. The largest land animals that ever lived, in fact. But they all died out in that extinction, paving the way for my kind — a class we call mammals — to take over.”
“In” “cred” “i” “ble,” said Hollus’s mouths. Sometimes he alternated whole words between his two speaking slits, and sometimes just syllables.
“How so?”
“How did you arrive at the dates for the extinctions?” he asked, ignoring my question.
“We assume that all uranium on Earth formed at the same time the planet did, then we measure the ratios of uranium-238 to its end decay product, lead-206, and of uranium-235 to its end decay product, lead-207. That tells us that our planet is 4.5 billion years old. We then—”
“Good,” said one mouth. And “good” confirmed the other. “Your dates should be accurate.” He paused. “You have not yet asked me where I am from.”
I felt like an idiot. He was right, of course; that probably should have been my first question. “Sorry. Where are you from?”
“From the third planet of the star you call Beta Hydri.”
I’d taken a couple of astronomy courses while doing my undergraduate geology degree, and I’d studied both Latin and Greek — handy tools for a paleontologist. “Hydri” was the genitive of Hydrus, the small water snake, a faint constellation close to the south celestial pole. And beta, of course, was the second letter of the Greek alphabet, meaning that Beta Hydri would be the second-brightest star in that constellation as seen from Earth. “And how far away is that?” I asked.
“Twenty-four of your light-years,” said Hollus. “But we did not come here directly. We have been traveling for some time now and visited seven other star systems before we came here. Our total journey so far has been 103 light-years.”
I nodded, still stunned, and then, realizing that I was doing what I’d done before, I said, “When I move my head up and down like this it means I agree, or go on, or okay.”
“I know that,” said Hollus. He clicked his two eyes together again. “This gesture means the same thing.” A brief silence. “Although I now have been to nine star systems, including this one and my home one, yours is only the third world on which we have found extant intelligent life. The first, of course, was my own, and the next was the second planet of Delta Pavonis, a star about twenty light-years from here but just 9.3 from my world.”
Delta Pavonis would be the fourth-brightest star in the constellation of Pavo, the peacock. Like Hydrus, I seemed to recall that it was only visible in the Southern Hemisphere. “Okay,” I said.
“There have also been five major mass extinctions in the history of my planet,” said Hollus. “Our year is longer than yours, but if you express the dates in Earth years, they occurred at roughly 440 million, 365 million, 225 million, 210 million, and 65 million years ago.”
I felt my jaw drop.
“And,” continued Hollus, “Delta Pavonis II has also experienced five mass extinctions. Their year is a little shorter than yours, but if you express the dates of the extinctions in Earth years, they also occurred at approximately 440, 365, 225, 210, and 65 million years ago.”
My head was swimming. I was hard enough talking to an alien, but an alien who was spouting nonsense was too much to take. “That can’t be right,” I said. “We know that the extinctions here were related to local phenomena. The end-of-the-Permian one was likely caused by a pole-to-pole glaciation, and the end-of-the-Cretaceous one seemed to be related to an impact of an asteroid from this solar system’s own asteroid belt.”
“We thought there were local explanations for the extinctions on our planet, too, and the Wreeds — our name for the sentient race of Delta Pavonis II — had explanations that seemed unique to their local circumstances, as well. It was a shock to discover that the dates of mass extinctions on our two worlds were the same. One or two of the five being similar could have been a coincidence, but all of them happening at the same time seemed impossible unless, of course, our earlier explanations for their causes were inaccurate or incomplete.”
“And so you came here to determine if Earth’s history coincides with yours?”
“In part,” said Hollus. “And it appears that it does.”
I shook my head. “I just don’t see how that can be.”
The alien gently put the Troodon skull down on my desk; he was clearly used to handling fossils with care. “Our incredulity matched yours initially,” he said. “But at least on my world and that of the Wreeds, it is more than just the dates that match. It is also the nature of the effects on the biosphere. The biggest mass extinction on all three worlds was the third — the one that on Earth defines the end of the Permian. Given what you have told me, it seems that almost all the biodiversity was eliminated on all three worlds at that time.
“Next, the event you assign to late in your Triassic apparently led to the domination of the top ecological niches by one class of animals. Here, it was the creatures you call dinosaurs; on my world, it was large ectothermic pentapeds.
“And the final mass extinction, the one you have referred to as occurring at the end of your Cretaceous, seems to have led to the shunting aside of that type and the move to the center of the class that now dominates. On this world it was mammals like you supplanting dinosaurs. On Beta Hydri III, it was endothermic octopeds like me taking centrality from the pentapeds. On Delta Pavonis II, viviparous forms took over ecological niches formerly dominated by egg layers.”
He paused. “At least, this is how it seems, based on what you have just told me. But I wish to examine your fossils to determine just how accurate this summary is.”
I shook my head in wonder. “I can’t think of any reason why evolutionary history should be similar on multiple worlds.”
“One reason is obvious,” said Hollus. He moved sideways a few steps; perhaps he was getting tired of supporting his own weight, although I couldn’t imagine what sort of chair he might use. “It could be that way because God wished it to be so.”
For some reason, I was surprised to hear the alien talking like that. Most of the scientists I know are either atheists or keep their religion to themselves — and Hollus had indeed said he was a scientist.
“That’s one explanation,” I said quietly.
“It is the most sensible. Do humans not subscribe to a principle that says the simplest explanation is the most preferable?”
I nodded. “We call it Occam’s razor.”
“The explanation that it was God’s will posits one cause for all the mass extinctions; that makes it preferable.”
“Well, I suppose, if . . . ” — dammitall, I know I should have just been polite, just nodded and smiled, the way I do when the occasional religious nut accosts me in the Dinosaur Gallery and demands to know how Noah’s flood fits in, but I felt I had to speak up — “. . . if you believe in God.”
Hollus’s eyestalks moved to what seemed to be their maximal extent, as if he was regarding me from both sides simultaneously. “Are you the most senior paleontologist at this institution?” he asked.
“I’m the department head, yes.”