But based on many of the emails we received (and the number of new queries from the Dear Science submission form) some of you are still skeptical about the whole idea of evolution. You've got questions — about where life came from, where it's going and how we know what path it took in the meantime. And that's great, because we've got answers.

Where are the transitional forms? Why don't we see evidence of a half-human, half-ape?

AD

AD

As we said at the start, technically you yourself are 100 percent ape — all humans are. But that's semantics. You're wondering about transitional forms, the oft-mentioned "missing links" that show some "primitive" traits and some "human" ones.

But we do see evidence of transitional forms in the evolutionary history of humans, and we're finding more and more of them all of the time. In 2009, scientists discovered the partial skeleton of a creature called Ardipithecus ramidus in Eastern Africa. "Ardi," as she's known, had thumblike big toes she used for climbing like a chimp, but her hips were also adapted to allow her to walk on two legs. Then there's Australopithecus afarensis, the species of Lucy: It combined the powerful lower jaws and long, strong arms of tree-climbing chimps with the small canines, larger brains and curved spines of bipedal humans. Homo habilis, which lived roughly 2.4 million to 1.4 million years ago, had some of the primitive traits of the earliest hominins — a pronounced brow ridge, large teeth. But it also showed signs of traits we consider essentially modern — it was capable of handling stone tools and had a bulge in the area of the brain responsible for speech.

The fossil record is not always as complete as we would like it to be. So much is up to chance — it's not easy to stay in one spot for millions of years, just waiting for the right keen-eyed paleontologist to come along. Animals disturb remains, bacteria break them down, floods may wash them away or earthquakes might shake them out of their geological context. But scientists have found more than enough evidence of transitional hominin forms to substantiate the story of human evolution.

AD

AD

Why aren't chimps evolving into smarter creatures that are using tools that their ancestors didn't?

This question assumes that natural selection should be pushing our great ape cousins — chimpanzees, bonobos, gorillas and orangutans — to be more like us. Not necessarily. Natural selection favors adaptations that best enable creatures to survive under the circumstances in which they live. That's why Emperor penguins evolved a warm, wind- and water-proof coat, camels have water-storing humps and whales have reinforced airways that keep their lungs from collapsing when they dive to great depths. These traits emerge because they're useful in those creatures' habitats. But if you moved a penguin to the savanna, or a whale to a shallow lake, and waited a few hundred thousand years, you might see them lose those qualities in favor of new ones, or go extinct altogether. That's life — marvelous and merciless all at once.

For humans, large brains, and the use of tools those brains enable, aided our survival during our evolution. But they are not inherently advantageous. Big brains require a lot of fuel; they also require big heads, which make giving birth dangerous and difficult. Hundreds of American women die in childbirth every year, according to the Centers for Disease Control and Prevention, and that's even with the benefit of modern medicine. Traits like that evolve only under specific pressures. Every species other than Homo sapiens, it seems, is better off without them.

That said, it's not entirely true that chimps aren't becoming smarter and using tools. In 2007, scientists reported in the Proceedings of the National Academy of Sciences that they had found stone tools used for breaking open nuts at a 4,300-year-old chimpanzee site in Ivory Coast. Chimps in the Ivorian rainforest still teach their young how to split nuts with rocks. Other nonhuman primates have been documented using sticks to dig for insects or honey, and in parts of Borneo, orangutans have been spotted using leaves as napkins to wipe their chins. Perhaps these species will become even more sophisticated users of tools over the years — the only way to find out is to wait and see. Nearly 4 million years passed between the time when our ancestors first started to drop down from the trees and when they started using the simplest of tools. Fire was not harnessed for 2 million more years, and farming was not invented until more than 700,000 years later. Even once environmental pressures start shaping the development of abilities takes time.

AD

AD

Why have we humans stopped evolving physically? What is the proof our present physical form is the best evolution can offer?

We haven't! Just this month, Jonathan Beauchamp, an economist at Harvard, published a study in PNAS on how natural selection has led to a shift in the genetic variants responsible for educational attainment and age of menarche among Americans born between 1931 and 1953. The effect is very small, he cautioned, but it's there.

And history is rife with more obvious examples: The mutation for lactose tolerance, which allows humans to keep drinking milk as adults, did not emerge until a few thousand years ago (long after we had started making cheese); now it's present in about half of all humans. People living at incredibly high altitudes have evolved adaptations that allow them to carry more oxygen in each red blood cell and take more breaths per minute, which keeps them from suffering from hypoxia in the thin mountain air. And in parts of the world where malaria is endemic, many populations have evolved resistance to the disease — sometimes at a terrible cost that would be nonsensical in any other part of the world, and therefore does not evolve in areas without malaria.

AD

AD

To suggest that our present physical form is "the best evolution can offer" shows how thoroughly human hubris shapes our understanding of science. As we mentioned earlier, humans are not the culmination of 4.5 billion years of evolution. Life didn't begin 3.5 billion years ago with the goal of ending at something like us; we are just one tiny, imperfect branch of a vast family tree. Our species could continue to evolve and change for many millennia to come, but we could just as easily go extinct, as have 99 percent of all life forms that came before us.

If evolution is real, where are the examples of it happening today?

About a century ago, a few British mosquitoes found themselves lost in the tunnels of the London Underground. Trapped beneath the earth, they foraged for food, searched for mates, and became so well-adapted to their subterranean habitat that they became genetically distinct. They eat different prey and require different climate conditions than other mosquitoes. When scientists tried to cross breed them with their above-ground counterparts, the eggs were infertile — evidence that a new species, Culex molestus, had emerged. Evolution in action, right beneath our feet.

AD

AD

For another example, turn to the Galapagos, the birthplace of our modern understanding of natural selection. Studies of the same finches that inspired Darwin a century and a half ago show that they're still evolving new traits to cope with environmental change. When a drought devastated one of the islands in the archipelago, one of the bird species living there evolved smaller beaks that allowed them to snatch up tiny seeds overlooked by other birds.

Evolution can happen so slowly you don't notice it, or so fast it takes your breath away. The kinds of changes that led to the emergence of Homo sapiens took millions of years. But we've also watched new species and new traits emerge right before our eyes. Sometimes, that's terrifying, such as when bacteria become antibiotic resistant in a matter of years. Other times, it's almost miraculous, such as when the tiny tomcod fish rapidly evolved to become tolerant of the toxins humans had been dumping in New York's Hudson River. Either way, it's an unstoppable process.

How does evolution explain the origins of life? Where did it all start?

AD

AD

We don't fully know the answer to this. Scientists' best guess is that life emerged from self-replicating strings of RNA that dwelled around hydrothermal vents in the deep sea or pools of chemically rich fluid. Over millions of years, these microscopic precursors to life evolved the ability to build proteins. The ones that built the best proteins were best able to survive and reproduce — natural selection at work! — and eventually some of their RNA mutated into the familiar double helix of genetic information we know as DNA. Those genomes got longer and more complicated, encoding information for the most successful types of RNA and protein molecules, and about 3.5 billion years ago, the first single-celled creatures emerged — the start of life as we know it.

We're still not certain that this explanation is the right one, but the evidence is compelling. Chemical reactions in labs have produced some nucleic acids — the building blocks of RNA and DNA — from the inorganic ingredients that would have existed on Earth. We've also found complex organic molecules encased in ice on meteorites. And tiny pathogens called "viroids," which are little more than unadorned loops of RNA, are thought to be relics of that primitive RNA world.

At the end of the day, we've still got more question than answers, and that's okay. It's what makes science so much fun. But when scientists call evolution a "theory," they don't mean there's a lack of evidence. A theory is the body of evidence used to describe something that can never be completely tested. Our understanding of the way gravity works is described in theories, too — and that's a phenomenon we witness about as often as the incredible results of natural selection seen on our own bodies.