One very important human ancestor was an ancient fish. Though it lived 375 million years ago, this fish called Tiktaalik had shoulders, elbows, legs, wrists, a neck and many other basic parts that eventually became part of us. This is the first story in our summer series The Human Edge, in which we examine how evolution created the most versatile creature on the planet.
It took him years of searching in the Canadian Arctic, but in 2004, Neil Shubin found the fossilized remains of what he thinks is one of our most important ancestors.
Turns out, it's a fish.
Shubin says his find, which he named Tiktaalik, represents an important evolutionary step, because it has the structures that will ultimately become parts of our human bodies. Shoulders, elbows, legs, a neck, a wrist -- they're all there in Tiktaalik.
"Everything that we have are versions of things that are seen in fish," says Shubin.
Of course, there are things that we have that Tiktaalik doesn't.
"We have a big brain, and portions of that big brain are not seen in Tiktaalik," says Shubin. "But the template, all the way down to the DNA that builds it, is already present in creatures like this."
Inside this fish, Shubin sees us.
"It's like peeling an onion," he says. "Layer after layer after layer is revealed to you. Like in a human body, the first layer is our primate history, the second layer is our mammal history, and on and on and on and on, until you get to the fundamental molecular and cellular machinery that makes our bodies and keeps are cells alive, and so forth."
Our Inner Yeast
In fact, not only are we related to an ancient fish, but many of the parts critical for making yeast are also critical for making us, says Gavin Sherlock, a geneticist at Stanford University.
"About one-third of the yeast genes have a direct equivalent version that still exists in humans," he says.
Sherlock says that not only do many of the same genes still exist in humans and yeast, but they're so similar that you can exchange one for the other.
"There are several hundred examples where you can knock out the yeast gene, put in the human equivalent, and it restores it back to normal," he says.
Think about it, he says: We have a lot in common with yeast. Yeast consume sugars like we do, yeast make hormones like we do, and yeast have sex -- not quite like we do, but sex.
Sex isn't just fun and games. Sexual reproduction is critical for stirring the genetic pot, speeding the evolution of endless forms most beautiful, from fruit flies to blue whales to humans.
Now yeast is a single-celled organism. We have trillions and trillions of cells in our bodies -- different kinds of cells, all fitting together. How did that happen?
The answer is at the Field Museum in Chicago.
How We Got A Body
Shubin points to a display case in an exhibition on evolution. "This tiny little diorama here, which you would just walk by, is arguably one of the most important ones for understanding our bodies," he says. "What you see is plastic fronds and jellyfish-like creatures in this primitive ocean, but it's here where single-celled creatures like bacteria and other microbes got together to make the first bodies."
And as time goes on, more forms emerge. Again, Shubin points at a display that's easy to miss. Inside is an ancient worm: It has a left and a right, a front and a back, a top and a bottom. These are the same coordinate axes as our bodies.
"In fact, we believe, if you look at the evolutionary history of these things, many of the genetic processes that make bodies like this and bodies like our own arose over 500 million years ago," says Shubin.
As Shubin and I walk through the exhibit, we see the results of tinkering with these genetic processes. Evolution brought fish, dinosaurs, mammals. Finally, we come to a familiar-looking 4-foot tall creature.
What Makes Humans Different
This is Lucy, an Australopithecus. She's more apelike than modern humans, but getting there. Despite Lucy's proximity to humans, she's clearly not human. Australopithicus went extinct.
On the way to us, something changed, and it was something more than just physical.
Shubin points to a cabinet across the room. Inside is a re-creation of a prehistoric human burial site. There's the skeleton of a woman who has been placed in the grave, surrounded by her jewelry.
"It's hard to look at this as a fossil anymore," says Shubin. "You look at this as a person who lived, and people loved this person enough to do this. And that's what changed."
Shubin says it's not a bone or a muscle or a gene that made us human. It was something else.
"The physiology and genetics made this possible. That's the template that made all this happen," he says. "But when was that spark, when was that moment? We don't know."
That moment that gave us the evolutionary edge that led to what we are today -- the species that buries its dead, builds museums, explores outer space. Shubin says it's the culture we built with our bones and muscles and brains that makes our species unique. Copyright 2010 National Public Radio. To see more, visit http://www.npr.org/.