Tell me what R and K species are, I said.
“Well, R animals have lots of babies, and they don’t take care of them,” The Girl said. “And K animals … are the opposite.”
Excellent. Which one do you think tends to be small and which one large? “R is small, and K is big,” she said.
Which one includes most amphibians and reptiles? “Definitely R,” said The Girl. “But there are a few frogs that take good care of their babies.” Yes, I said, and some mammals lean more to the R side – these aren’t absolute categories, just general tendencies. There’s a spectrum in-between, and a lot of living things that don’t fit either one. Which one tends to include animals lower down on the food chain, and which one further up?
“Most things lower down would be R,” she said. Right, and apex predators like polar bears tend to be K. What about stable and unstable environments, places where the climate or temperature stays the same or changes a lot?
She wasn’t sure, but I explained that R animals tend to do better in unstable or extreme environments – when a new habitat opens up, like an island volcano appearing out of the water, the R species populate it first.
“Are we the K-est of Ks?” The Girl asked. Just about, I said – we have a very long childhood, as there’s so much we need to learn to take proper care of the world. Even with us, though, some people are a bit more R than others.
“They’re more like fish?” The Girl asked. “I mean, they can’t be completely like fish, for fish have thousands of babies.”
Yes, and thankfully human females can’t, I said, but people used to have a lot more – five, seven, even fifteen children in a family, because so many would die in childhood. Some people still do have big families.
She nodded – in a land where recorded history goes back more than two thousand years, you can imagine there are lots of headstones around.
“How far are we from fish?” she asked, changing the subject. “Cause you know that our ear bones are the same bones that used to hold the gills up in fish?”
You’re right, I said. I’d guess about 400 million years – that’s when different kinds of fish were branching out and filling the seas. But that brings us to an interesting point – one of the most famous biologists who ever lived, the scientist Stephen Jay Gould, once said that there is no such thing as a fish.
“Is it because fish aren’t really mostly fish?” she asked.
I wasn’t sure what she meant. Not quite, I said – he meant that there’s no family of animals called “fish” – they branched out into many families, and a few of them became amphibians and so on, but the others are separated by 400 million years or so. A coelacanth might be more closely related to a gorilla than it is to a tuna or a shark, and of course many of the other things we call fish, like shellfish, aren’t even vertebrates. We just call all these things fish because they swim, like taking all the things that fly – bats, dragonflies, robins – and calling them birds.
“Is that just fish, or is that the same with other things too?” she asked.
The same goes for other things, I said – in terms of genetic families, you could say there’s no such thing as a tree, or a reptile. They’re not all related – they’re just handy labels for types of bodies.
“I thought it might be because … you know how none of us are mostly us? How most of the cells in our body aren’t really the ‘us’ cells?” she asked.
Suddenly I understood. This had been a previous lesson – we contain more bacteria in our body than we do “our own” cells, the cells with our own DNA. I heard a biologist once say that if our own cells were packed together in our body, they would fill one of our legs below the knee.
If that sounds alarming, keep in mind that the bacteria are not only benign, but mostly necessary – they fill your innards and digest your food, for example. They are left over from the earliest ages of the Earth, made for the unrecognisable planet that existed before the sky turned blue with oxygen, and are many times older than bones or shells. Now they survive by hiding inside us, keeping us alive.
This information seems to unsettle many adults, but as I had hoped, it fascinated The Girl, and she has brought it up proudly many times since. She was delighted to find that her child’s ribs contain the mengerie of an alien world, and that beneath her baby skin lie a multitude of ancient beings.