Dear Science: Why are there no hominins left on Earth? If evolution is ongoing and species are always changing and adapting, shouldn't we see new human-like species evolving from apes, even if the old ones died out?

Here's what science has to say:

We hate to be the ones to break it to you, but you are an ape.

So were the Neanderthals, the Hobbits, Lucy the Australopithecus, the Taung child and Peking man. And while we're at it, so are orangutans, gorillas, bonobos and chimpanzees. All of us evolved from a common ancestor that lived about 14 million years ago, and together we make up the taxonomic family Hominidae. Also known as hominids. Also known as great apes.

And there are hominins left on Earth — us. "Hominin" is the the technical term for archaic and modern humans — that is, creatures that are more closely related to us than they are to gorillas and chimps. (We know, the terminology can be confusing. Bring it up with the paleoanthropologists.) And to explain why we are the only ones around — for now, at least — you have to think about how evolution works.

First of all, the creatures we call apes are our cousins, not our ancestors. Which would make it very hard for them to evolve into something like us.

"Asking why an archaic human isn't evolving from gorillas today is like asking why the children of your cousins don't look more like you," said Matt Tocheri, an anthropology professor at Lakehead University and a researcher in the National Museum of Natural History's Human Origins Program. "Those creatures have been on their own lineage for 10 million years. You can't go back up that lineage and back down again."

Even if chimpanzees could suddenly develop the traits of an Australopithecus, they probably wouldn't want to.

It's easy to think about evolution as a linear, progressive drive toward greater and greater complexity, something that started with single-celled amoebas and ended with us. But evolution doesn't have a destination, and even if it did, humans are certainly not it. In many cases, evolution tends to favor simplicity. That's why creatures that live in caves lose their eyes, and whales — which are descended from terrestrial mammals — have almost no leg bones. Not even intelligence is sacred: sea urchins, which have no central nervous system, evolved from an ancestor with a brain.

"Evolution is about survival under particular conditions, and random mutations," says Nina Jablonski, a paleoanthropologist at Penn State. "There's a big element of chance and certainly no element of direction. ... Living things are just trying to adapt to the contingencies of life in their environment."

The diversity of hominins during the earliest stages of human evolution showed how several species tried to do that. For example, it's thought that Australopithecus afarensis (Lucy's species) evolved human-like hips that let them walk on two feet because it let them carry things — a useful skill for collecting food on the savanna. Paranthropus robustus had a powerful jaw for chewing the tough, fibrous foods available in their dry environment. Homo habilis had a relatively huge brain that helped them make early stone tools. Australopithecus boisei's massive molars let him dine on mostly nuts and seeds.

But as hominins' tool use got more sophisticated, their ecological niches expanded. Species didn't have to choose between having big molars for chewing seeds and sharp canines for ripping meat — with tools, they could partially break the food before eating it and consume both.

"As the technological complexity of humans increases ... one species with tools is able to do more than two or three species could in the past," Jablonski said. Homo species with larger brains and smaller teeth were more ecologically successful, so evolution favored those groups. The development of language added to our ancestors' evolutionary toolkit, helping them hunt, travel, anticipate and avoid threats. By the time Homo sapiens arose roughly 200,000 years ago, they were able to survive in almost any environment, under any circumstances.

"They're able to adapt to dramatically changing climates all over the world and disperse much more widely because they can make a bunch of different stuff," Jablonski said. "So when we see the final extinction of the Neanderthals or the funny little Hobbit guys in Indonesia" — two Homo species that lived at the same time as modern humans — "as a result of dramatic climatic changes, modern humans with this incredible toolkit and ability to duck and dive around the problems of the world, they're able to survive."

Vanderbilt researcher Tony Capra talks about his research regarding the presence of Neanderthal genes in the genomes of a significant population of adults of European ancestry. (Vanderbilt News and Communications)

In other words, modern humans were able to outlive other hominins because we were equipped to exploit multiple ecological niches under the particular circumstances in which we lived. Had the climate not changed 30,000 years ago, or if it had changed differently, the Neanderthals might have survived instead of Homo sapiens.

That's important to remember when we ask why our ape cousins aren't evolving the traits that characterize humans.

Modern great apes live in heavily forested environments where the ability to climb trees is a big bonus — so they have no need for human bipedalism. Creatures like chimpanzees and bonobos are capable of building nests, using rudimentary tools, appreciating beauty, and perhaps even mourning their dead — without our energy-guzzling big brains.

"When we look at our ape relatives today, they're just fine being ape-y," Jablonski said. "They're doing their chimp stuff, their orangutan stuff, their gorilla stuff; they don't need to be more human-like because they're surviving perfectly."

"Of course," she added, "that's predicated on the hope that modern humans don't chop down their forest completely and entirely deprive them of their natural habitat. But that's a different issue."