15 February, 2009

Sunday Sensational Science

Lucy's Legacy

Lucy got us on our feet and thinking.

That's no small thing, although she was tiny - about the size of a seven year-old. She was a big deal here in Seattle this Valentine's Day, when a passel of people walked over to see her at the Pacific Science Center, and got to thinking about her legacy. I'll have a lot more to say about her in a future installment. For now, we're going to take a photo journey through the pieces of our past the exhibit touched on.

The exhibit has a replica of the Laetoli Footprints. 3.6 million years ago, some of Lucy's kin, Australopithecus afarensis, walked through volcanic ash, and left us a record of their bipedal meanderings. There's something extraordinary in seeing the footsteps of those who walked before us. Bipedal posture seems so uniquely human that it leaves an immediate impression. Other becomes us in an instant. Suddenly, you can picture yourself walking side-by-side with a little ape-like ancestor who wouldn't have to strain her back to hold your hand. There's something special in that. It brings paleoanthropology alive.

Lucy almost wasn't found. When you see fossils in a museum, they're already extracted from their rock matrix, easy to see, sometimes stained dark. You begin to think that the dark brown patina is the actual color, but fossils are products of their surroundings, and blend in. And if it wasn't for the instincts of a sharp-eyed paleontologist, Lucy wouldn't be here in Seattle today:
Then, on the morning of November 24, 1974,[3] near the Awash River, Johanson abandoned a plan to update his field notes and joined graduate student, Tom Gray from Texas State, in taking their Land Rover to Locality 162 to search for bone fossils.[9] Both Johanson and Gray spent a couple of hours on the increasingly hot arid plains, surveying the dusty terrain, then Johanson decided on a hunch to make a small detour on their way back to the Land Rover to look at the bottom of a small gulley that had been checked at least twice before by other workers. At first sight there was virtually no bone in the gulley, but as they turned to leave, a fossil caught Johanson's eye; an arm bone fragment lying on the slope.
That was Lucy, shining in the sun.

In the end, they discovered 40% of her, an incredible find. She remains one of the most complete ancient hominid skeletons ever found, and she's taught us a lot about how we came to be us.
"She occupies a pivotal place on the human family tree," said Donald Johanson, the American paleoanthropologist who, with his colleagues, discovered the fossil in 1974 near the northern Ethiopian community of Hadar. "We now know that one of the first significant things our ancestors did was to stand up, to walk on two feet instead of four."
The exhibit helps resolve a debate that's been raging since well before I was born: was it the brain or the bipedalism that came first? Lucy says we got on our own two feet before we started on the big brain portion of our program. Below, you'll see a chart showing the brain sizes of many of our ancestors and close cousins. Lucy's posture wasn't terribly different from ours, but her brain capacity certainly was. A modern human boasts a cranial capacity of about 1100-1700 cc. Hers would have been between 375-500 cc. Einstein she wasn't - at least, not compared to us.

Upright posture, it seems, came first, freeing our hands for other tasks. And those free hands got busy manipulating the world, which helped lead to bigger and bigger brains.

I wasn't thinking all that much about brains, although I'd had my fun with the display of water bottles that could be flipped upside-down to drain into the skulls of a chimpanzee, A. afarensis, and homo sapiens. It's the knees that caught my attention. I didn't know a damned thing about knee angles until I saw a display up against the wall demonstrating what upright posture does to the shape of the knee. It's fascinating:

Femurs of upright walkers and ape

Leg of ape
Quadrupedal animals like apes, have femurs in which the ball joint, the part that joins the pelvis, sits directly over the inside of the knee. The angle subtended by the femur at the knee in quadrupedal walkers is less than that of bipedal walkers.

Leg of Australopithecus afarensis
This diagram shows the femur with the same shape and structure as that of modern humans, but it is a little shorter. It subtends the same angle at the knee as that of a modern human and the inner bump of the knee joint is larger than the outer one. This shows that this hominin was also a bipedal walker.

Leg of modern human
This modern Homo sapiens bone shows the structure of the femur of an upright walker or bipedal animal. The ball joint, the part that joins the pelvis, sits directly over the outside of the knee. The angle subtended by the femur at the knee in bipedal walkers is greater than that of quadrupedal walkers. This results in the inner bump of the knee joint being longer than the outer bump.

Pretty amazing, isn't that just?

That posture does interesting things to the pelvis. And hers looks remarkably like one of ours, aside from not having to be so wide to accommodate big-brained babies. You can see the similarity in the picture at right:

Comparison of three female pelves over the course of the last three million years. The pelvis of "Lucy" (Australopithecus afarensis, around 3.2 million years old) on the left, the new Gona pelvis (Homo erectus, about 1.2-1.3 million years old) in the middle, and a modern human female (Homo sapiens) on the right.
That's a spectacularly clear evolutionary progression right there, isn't it just? It's really hard to stare at Lucy's hips, pelvis and knees without being dumbstruck by the similarities.

But that's nothing compared to walking up a long, sloping corridor filled with hominid skulls, and realizing that some of these folks would've looked a lot like us.

Kneeling down, looking in the eye sockets of some of those skulls, seeing the shapes become so close to our own, gave me an eerie thrill. I could see the sutures in a Neanderthal's skull. I could see the softening of the brow ridges as time went on, the increase in brain size, the changes in cheeks and jaw. We think of our ancestors and cousins as ape-like brutes, but a lot of them, like Neanderthal, were nearly identical. Seeing those skulls all lined up makes you realize how close we are to where we came from.

And, strangely enough, little Lucy's skull, chimp-like as it is, also has some incredibly human features. Her jaw, her teeth, aren't as different from us as you might expect.

There's a sculpture of her there, nearly the first thing you see when you walk into the room where her bones lie spotlit in the dark. She stares over her shoulder, almost coquettish, and utterly adorable. This image to the right was taken at a bad angle - it doesn't capture the slight smile, the serene joy in her expression. She's definitely an ape, still, but she strongly foreshadows the humans to come.

It's a delightful sculpture that makes Lucy more than just a collection of remarkable bones. You want to slip your hand into hers, let her tug you into her world. You get the feeling that the two of you would relate on a level a little closer than humans and chimps. She has a lot to tell us, and in a way, it's too bad that all we can listen to are her bones.

But what incredible bones they are.

The first thing that strikes you is how small they are. The rib bones are no wider than my finger. Her vertebrae are tiny. And her pelvis is a delicate, beautifully curved structure that doesn't look strong enough to bear an upright body. She was under four feet tall, which is a bit of a jolt when compared to the size of her legend.

The second thing you notice is her pallor. The bones are a silvery-tan, with only hints of darker brown here and there, almost shimmering against the stark black of the case they have her in. It's incredible that those pale bones were ever found in the pale dust of the Ethiopian desert.

It does inspire awe, gazing down at bones that are nearly four million years old, that have so many human characteristics that the ape-like ones fade into the background. Even an amateur like me can see how much she had to say about who we are, what we were, and how we got from there to here.

She's an inspiration. She's glorious. She's earned both of her names: Dinkenesh, which means "Wonderful one," and Lucy - because, for all that she's lying under glass in a museum, it truly seems she is "Lucy in the Sky with Diamonds."

Go see her, should you get the chance. She'll tell you something wonderful.

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