“Sorry?” said Ponter.
“It’s a pun; a play on words. You know: ‘magnetic moment’—the product of the distance between a magnet’s poles and the strength of either pole.”
“Ah,” said Ponter. This Gliksin obsession with word play…he would never understand it.
Arnold looked disappointed. “Anyway,” he said, “I’m surprised that your magnetic field collapsed before ours did. I mean, I understand the Benoît modeclass="underline" that this universe split from your universe forty thousand years ago, at the dawn of consciousness. Fine. But I can’t see how anything your people or mine might have done in the last four hundred centuries could have possibly affected the geodynamo.”
“It is puzzling,” agreed Ponter.
Arnold clambered off his chair and rose to his feet. “Still, because of it, you’ve been able to satisfy my particular concern better than I would have thought possible.”
Ponter nodded. “I am glad. You should indeed—how would you phrase it?—you should sail effortlessly through the period of magnetic-field collapse.” He blinked. “After all, we certainly did.”
Chapter Twelve
Mary tried to concentrate on her work, but her thoughts kept turning to Ponter—not surprisingly, she supposed, since Ponter’s DNA was precisely what she was working on.
Mary cringed every time she read a popular article that tried to explain why mitochondrial DNA is only inherited from the maternal line. The explanation usually given was that only the heads of sperm penetrate eggs, and only the midsections and tails of sperm contain mitochondria. But although it was true that mitochondria were indeed deployed that way in sperm, it wasn’t true that only the head made it into the ovum. Microscopy and DNA analyses both proved that mtDNA from the sperm’s midsection does end up in fertilized mammalian eggs. The truth was no one knew why the paternal mitochondrial DNA isn’t incorporated into the zygote the way maternal mitochondrial DNA is; for some reason it just disappears, and the explanation that it had never gotten in there in the first place was nice and pat, but absolutely not true.
Still, since there were thousands of mitochondria in each cell, and only one nucleus, it was much easier to recover mitochondrial rather than nuclear DNA from ancient specimens. No nuclear DNA had ever been extracted from any of the Neanderthal fossils known from Mary’s Earth, and so Mary had been concentrating on studying Ponter’s mitochondrial DNA, comparing and contrasting it with Gliksin mtDNA. But there didn’t seem to be any one sequence she could point to that was present in Ponter and the known fossil Neanderthal mitochondrial DNA, but in none of the Gliksins, or vice versa.
And so Mary at last turned her attention to Ponter’s nuclear DNA. She’d thought it would be even more difficult to find a difference there, and indeed, despite much searching, she hadn’t found any sequence of nucleotides that was reliably different between Neanderthals and Homo sapiens sapiens; all her primers matched strings on DNA from both kinds of humans.
Bored and frustrated, waiting for Ponter to be released from quarantine, waiting to renew their friendship, Mary decided to make a karyotype of Neanderthal DNA. That meant culturing some of Ponter’s cells to the point where they were about to divide (since that’s the only time that chromosomes become visible), then exposing them to colchicine to immobilize the chromosomes at that stage. Once that was done, Mary stained the cells—the word “chromosomes,” after all, meant “colored bodies,” referring to their tendency to easily pick up dye. She then sorted the chromosomes in descending order of size, which was the usual sequence for numbering them. Ponter was male, and so had both an X and a Y chromosome, and, just as in a male of Mary’s kind, the Y was only about one-third the size of the X.
Mary arrayed all the pairs, photographed them, and printed out the photo on an Epson inkjet printer. She then started labeling the pairs, beginning with the longest, and working her way to the shortest: 1, 2, 3…
It was straightforward work, the kind of exercise she’d put her cytogenetics students through each year. Her mind wandered a bit while she was doing it: she found herself thinking about Ponter and Adikor and mammoths and a world without agriculture and…
Damn!
She’d obviously screwed up somehow, since Ponter’s X and Y chromosomes were the twenty-fourth pair, not the twenty-third.
Unless…
My God, unless he actually had three chromosome 21s—in which case he, and presumably all his people, had what in her kind produced Down’s syndrome. That made some sense; those with Down’s had an array of facial morphologies that differed from other humans, and—
Good grief, thought Mary, could it be so simple? Down’s sufferers did have an increased incidence of leukemia…and wasn’t that what Ponter said had killed his wife? Also, Down’s syndrome was associated with abnormal levels of thyroid hormones, and those were well-known to affect morphology—especially facial morphology. Could it be that Ponter’s people all had trisomy 21—one small change, manifesting itself slightly differently in them than it did in Homo sapiens sapiens, accounting for all the differences between the two kinds of humans?
But no. No, that didn’t make sense. Principal among Down’s effects, at least in Homo sapiens sapiens, was an under development of muscle tone; Ponter’s people had exactly the opposite condition.
And, besides, Mary had spread out an even number of chromosomes in front of her; Down’s syndrome resulted from an odd number. Unless she’d accidentally brought some chromosomes in from another cell, it appeared that Ponter did indeed have twenty-four pairs, and…
Oh, my God, thought Mary. Oh, my God.
It was even more simple than she’d thought.
Yes, yes, yes!
She had it!
She had the answer.
Homo sapiens sapiens had twenty-three pairs of chromosomes. But their nearest relatives, at least on this Earth, were the two species of chimpanzees, and—
And both species of chimps had twenty-four pairs of chromosomes.
Genus Pan (the chimps) and Genus Homo (humans of all types, past and present) shared a common ancestor. Despite the popular fallacy that humans had evolved from apes, in fact, apes and humans were cousins. The common ancestor—the elusive missing link, not yet conclusively identified in the fossil record—had existed, according to studies of the genetic divergence between humans and apes, something like five million years ago in Africa.
Since chimps had twenty-four pairs of chromosomes and humans had twenty-three, it was anyone’s guess as to what number the common ancestor had possessed. If it had had twenty-three, well, then, sometime after the ape-human split, one chromosome must have become two in the chimp line. If, on the other hand, it had had twenty-four, then two chromosomes must have fused together somewhere along the Homo line.
Until today—until right now, until this very second—no one on Mary’s Earth had known for sure which scenario was correct. But now it was crystal clear: common chimps had twenty-four pairs of chromosomes; bonobos—the other kind of chimp—had twenty-four as well. And now Mary knew that Neanderthals also had an even two dozen. The consolidation of two chromosomes into one had happened long after the ape-human split; indeed, it had happened sometime after the Homo branch had bifurcated into the two lines she was now studying, only a couple of hundred thousand years ago.
That was why Ponter’s people still had the huge strength of apes, rather than the puniness of humans. That was why they had ape physiognomy, with browridges and no chins. Genetically, they were apelike, at least in chromosome count. And something about the fusing of two chromosomes—it was numbers two and three, Mary knew, from studies of primate genetics she’d read years before—had caused the morphological differences that gave rise to the adult human form.