For living species, John Damuth, a biologist at the University of California, Santa Barbara, came up with a mathematical relationship, now known as Damuth’s law, between the average body mass of an animal and its expected population density.

The relationship is not universal but generally holds for large classes of animals like lizards or meat-eating mammals. So, for Tyrannosaurus rex, they had to not only plug in the weight of the dinosaur — about six tons, give or take a few — but also derive other numbers in the law.

Mention of Charles Darwin, for most, conjures up images of intrepid Victorian sea voyages, giant tortoises and Galapagos finches. Few of us associate Darwin with plant sex. That honor tends to go to his grandfather, Erasmus Darwin, who wrote erotic poems on the topic.

Although Darwin’s On the Origin of Species, which describes his theory of evolution by natural selection, has eclipsed all his other research, his career continued for over two decades after the landmark work’s publication. Much of the aging naturalist’s time was spent studying botany, and his research produced discoveries that, had he not become famous for natural selection, would have made him a well-known botanist. (...)

British naturalist Charles Darwin got it right, but maybe we got Darwin wrong.

Most people assume that Darwin was talking about physical strength when referring to “survival of the fittest,” meaning that a tougher, more resilient species always will win out over its weaker counterparts. But what if he didn’t mean that at all?

Scientists Brian Hare and Vanessa Woods, both researchers at Duke University’s Center for Cognitive Neuroscience, believe something else has been at work among species that have thrived throughout history, successfully reproducing to sustain themselves, and it has nothing to do with beating up the competition.

Their new book, “Survival of the Friendliest: Understanding Our Origins and Rediscovering Our Common Humanity,” posits that friendly partnerships among species and shared humanity have worked throughout centuries to ensure successful evolution. Species endure — humans, other animals and plants — they write, based on friendliness, partnership and communication. And they point to many life examples of cooperation and sociability to prove it.

“Survival of fittest, which is what everyone has in mind as evolution and natural selection, has done the most harm of any folk theory that has penetrated society,” Hare says. “People think of it as strong alpha males who deserve to win. That’s not what Darwin suggested, or what has been demonstrated. The most successful strategy in life is friendliness and cooperation, and we see it again and again.”

“Dogs are exhibit A,” he says. “They are the extremely friendly descendants of wolves. They were attracted to humans and became friendly to humans, and changed their behavior, appearance and developmental makeup. Sadly, their close relative, the wolf, is threatened and endangered in the few places where they live, whereas there are hundreds of millions of dogs. Dogs were the population of wolves that decided to rely on humans — rather than hunting — and that population won big.” (...)

(...) This evolutionary two-step — first spillover, then adaptation to the new host — is probably characteristic of most viruses as they shift hosts, says Daniel Streicker, a viral ecologist at the University of Glasgow. If so, emerging viruses probably pass through a “silent period” immediately after a host shift, in which the virus barely scrapes by, teetering on the brink of extinction until it acquires the mutations needed for an epidemic to bloom.

Streicker sees this in studies of rabies in bats — which is a good model for studying the evolution of emerging viruses, he says, since the rabies virus has jumped between different bat species many times. He and his colleagues looked at decades’ worth of genetic sequence data for rabies viruses that had undergone such host shifts. Since larger populations contain more genetic variants than smaller populations do, measuring genetic diversity in their samples enabled the scientists to estimate how widespread the virus was at any given time.

The team found that almost none of the 13 viral strains they studied took off immediately after switching to a new bat species. Instead, the viruses eked out a marginal existence for years to decades before they acquired the mutations — of as yet unknown function — that allowed them to burst out to epidemic levels. Not surprisingly, the viruses that emerged the fastest were those that needed the fewest genetic changes to blossom.

SARS-CoV-2 probably passed through a similar tenuous phase before it acquired the key adaptations that allowed it to flourish, perhaps the mutation to the polybasic cleavage site, perhaps others not yet identified. In any case, says Colin Parrish, a virologist at Cornell University who studies host shifts, “by the time the first person in Wuhan had been identified with coronavirus, it had probably been in people for a while.”

It was our bad luck that SARS-CoV-2 adapted successfully. Many viruses that spill over to humans never do. About 220 to 250 viruses are known to infect people, but only about half are transmissible — many only weakly — from one person to another, says Jemma Geoghegan, an evolutionary virologist at the University of Otago, New Zealand. The rest are dead-end infections. Half is a generous estimate, she adds, since many other spillover events probably fizzle out before they can even be counted. (...)