Who are we? Where did we come from? Where are we going? Why are we so different from all other animals?
What little is known in the way of answers to those questions-and many more-cannot be addressed in one article, or even a series. From those first tentative proto-human footprints captured in ancient volcanic ash millions of years ago, across the burning deserts of Arabia, through the frozen steppes of ice-age Europe, to a New World full of hope and danger, to the crystal clear imprint of mankind's first steps on the moon; it's quite a tale of monsters eluded, catastrophes averted, and challenges faced down! Ours is an epic journey through time far beyond the scope of this series. But, I invite you to walk with me as only we humans can, across the ages and epochs to learn a bit about ourselves and to explore some of what we know. This is the story of the walking apes: The Hominids.
Hominid footprint: Laetoli Africa, 3.5 million years ago. Hominid footprint: The Moon, 1969 AD
Our walk through the past begins in the late
Miocene, seven-million years ago: The earth then was hot, wet, and wild! Luxuriant rain forests spanned entire continents forming a vast, lush-green triple layered carpet. Mighty evergreens soared into the sky for almost 200 feet, drench in lianas vines. Rubber trees and ferns dripping with humidity pulsed beneath those mighty sentinels, greedily lapping up scraps of sunlight here and there, while occasionally allowing some rays through to the impassable tangle of bushes below. A determined tree dwelling explorer could have traveled from the East coast of China to the western coast of Spain, without ever touching the jungle floor. Twenty-pound Micropithecus flashed through the continuous canopies with a dexterity modern day gymnasts would envy, while half-ton Gigantopithecus foraged on the quiet dark forest floor. It was a steamy, primate paradise, with over one-hundred varieties of great ape alone. Each one was as different from one another as Gorillas are from Gibbons; such diversity, such beauty of form and function, such an array of intelligence.
But the Miocene passed into geological history, most of the apes followed suit, and the great jungles broke up. Yet one lineage profited immensely from the shifting fortunes. The one that learned to walk.
The term ape is not a precise scientific term, but it generally refers to a higher primate which has no tail. Using that loose definition, today there are five living species of such creatures making up perhaps ten subspecies: Gorillas, Orangutans, Gibbons, Chimpanzees, and Humans. These are the handful of survivors which made it safely into our own era. Out of all five we are the only extant hominids, the only living apes who are obligated to walk upright on two legs.
All species are special and unique in their own way. Spiders are masters of the ambush and snare, cetaceans and bats exhibit a facility for sonar and spatial visualization surpassing a Trident Submarine with the most sophisticated gadgetry, marine snails shoot darts of exotic neurotoxins into prey at the speed of sound, the Venus flytrap craftily lures small insects into a deadly, leafy, embrace. But of all the species that have roamed the planet, we and we alone have been endowed with a degree of intelligence and technical proficiency which is light-years ahead of the nearest rival.
It's tempting then to imagine our present intellect is what identifies us as human. If so you might be making the same natural mistake early paleoanthropologists made when they went looking for the classical missing link of the early twentieth century-big brained, slouched over, knuckle-walking, killer ape. But that was a quest that would never be fulfilled. There was no such creature.
It was an earlier adaptation which really set us apart from our ape cousins and sent us on the road to sentience. It was with our distant ancestors as they stumbled out of the primeval paradise onto dry plains. It's an adaptation we think of as so natural, so easy, we scarcely give it a thought as an adult-until we can no longer do it. A toddler acquires and masters the skill without any conscious effort. Although it seems obvious to us, second nature, we are the only large mammal known which travels the way we do. Some birds and dinosaurs came close, but they were teeter-totters balanced over a pelvic fulcrum by torso and tail. Hominids walk and run daintily on two feet, our precious swollen brains precariously balanced on a pogo-stick spine far from the ground.
A hominid possesses several distinctive features. The bones may not look particularly revealing to the layperson. But once you've handled the casts a few times and know what to look for, there can be no doubt.
Teeth can at least tell us if we're dealing with an ape. One of the most unique morphological traits of all such creatures is the Y-5 molar pattern. This distinctive architecture of five raised cusps shown above, inlaid with a distorted branching Y, is unique in all the animal kingdom to our close apish kin.
Under the skull of every animal is a large hole, called the Foramen Magnum, through which the spinal cord exits the braincase. In hominids, this hole is placed directly beneath the crania (Left image) as opposed to behind it such as we see in gorillas or chimps (Right image). Other foramina such as the passage for the jugular vein are similarly distinct between bipeds and quadrupeds. The muscles which attach to the back of the hominid skull insert at unique points compared to the knuckle-walking quadrupeds. These gentle bumps and ridges, called processes or condyles, are easily discerned as the exclusive property of an upright walking ape by any undergraduate anthropology major.
Look carefully at the human skeleton on the far left, next to a gorilla, chimpanzee, orangutan, and the elvish Gibbon, respectively. Compare and contrast the rest of the skeleton below the skull; the post crania we would say in anthropology. The hominid spine has a sweeping double curve, no other animal known even comes close to that spinal morphology. Each lumber vertebra is shorter on the backside and longer on the inside to produce that distinctive lumber curve and those lumber vertebra are robust compared to the lower vertebra in the quadrupedal apes; they have to be to support the load directly above them. The hominid pelvis is compact and hints of a shallow cone with an open apex pointing to the ground. Our femurs emerge from the hip sockets and angle inward to the knee so as to be directly underneath the center of gravity. The end of the femur articulates on top of the tibia in an exclusive manner not found in the other apes. The tibia and fibula are long, ending in modified feet radically reworked by evolution. The hominid legs are much longer than the arms.
If you have even a tiny piece of skull or other bone showing those signs, and a bit of one single molar with a Y-5 pattern, both from the same individual or species and from the proper time, you can be reasonably sure you have a hominid. And for the first half of our 8 million year journey out of the primeval Miocene Forest onto the new grasslands, that's about all we do have.
For those of you interested in evolution beyond us foolish humans or even the mere terrestrial ... The Discovery Channel is teeming up with Animal Planet, along with some stellar names in science, to present Alien Planet.
This program features a hypothetical future world orbiting a nearby star, explored by unamanned craft from earth, called Darwin IV, in which evolutionary biology has taken some surprising twists and turns. The Gallery of Aliens looks like lots of fun for kids as well as the whole family, who values science and imagination. For added fun, invite the fundamentalist neighbors over to watch along! Alien Planet airs this Saturday, May 14, at 8:00 PM Eastern Time
Our walk now takes us first through a blank tangle of scrub leading out of a Miocene forest onto a Pliocene plain. The trail of evidence here is overgrown and hard to make out. Unfortunately, specimens preserved by fossilization are few and far between in a steamy jungle environment. The soil is acidic and the rainforest notoriously rich in scavengers. Little is left to preserve when the predators and carrion eaters are done dining on the remains. We don't know why or how our ancestors hit upon walking like we do. It was likely in response to a decrease in the great forests of the Miocene. But why didn't they adopt a more traditional approach, such as that used by baboons or prairie dogs? Such a form of locomotion as ours come replete with back problems, fallen arches, headaches, circulation problems; the list of maladies goes on and on. Animals built like a sawhorse with four sturdy legs and a backbone strung like a suspension bridge are surely more intelligently designed. The price we pay for bipedalism is high, so high the reward must have been substantial indeed.
We know what to look for, we know how to tell a bipedal ape from a quadruped, the question is why and how did bipedalism develop at all?
There are a number of ideas, but the truth is no one knows for sure and we may never know with confidence. Perhaps it begin modestly as a temporary standing posture so as to better to see food sources and predators over the tall grass and went from there. Walking also frees our hands and allows us to take larger quantities of food back to our forest enclave of awaiting females and young-a potentially powerful selective advantage. Standing and walking takes less energy than four legged gaits, so it's more efficient over longer distances, over plains and grasslands, or between stands of forest refuge stubbornly resistant to the change which knocked off the global rain forests at the end of Miocene. And standing vertically helps keeps that big hominid brains from overheating, staying cooler by getting the head away from the warm ground and up into the open air.
It could have been any or all of those, or a host of factors we know nothing about may have played a prominent role. Fossils can only tell us when we became bipedal, and that's if we're lucky enough to find the right ones. Even the best specimens cannot tell us much about why. The events surrounding why and how our ancestor went from quadrupeds to bipeds are probably too remote in time and too complex to ever determine with precision merely from a skeleton. And in practice, hominid fossil remains are usually limited to mostly the hardest bone and enamel. The parts which preserve the best from a carcass after the jackals and worms have finished their ghastly business. Teeth and jaws mostly, some pelvic or limb bones from time to time, and bits of skull.
The first walking ape denizen you and I might see on our short stroll is the oldest known probable hominid on earth dating to about 7 million years ago. Sahelanthropus tchadensis, nicknamed Toumia, was found in Chad in 2001 and consist of a remarkably complete hominid skull. But the critical portion surrounding the Foramen Magnum is missing. Bipedalism is inferred from the occipital muscle attachment points. Whether or not the creature was an obligatory biped is unclear.
Following a gap filled only by a few fossil teeth and other bits of bone representing a poorly understood possible hominid called Orrorin Tungenesis (Circa 6 MYA), the next likely hominid crops up in our path about four and half million years ago. Ardipethicus, of which two species are suspected. Ramidus is the better described and the case for obligate bipedalism therein can be summed up as:
Tim White "Some portions of the basicranium are preserved (occipital and temporal portions), and they provide some important information. The occipital condyles are as small as the smallest known australopithecine adults, which indicates a very small size for the specimen (ARA-VP 1/500). The size of the condyles corresponds to the force exerted by nuchal muscles, which is correlated to body size. Also, the position of the foramen magnum is very anterior to the cranial base, similar to the placement in other australopithecines. This feature, along with a deep digastric sulcus indicating a shortened cranial base, strongly indicate that A. ramidus held its head upright like other australopithecines can.
Again the scraps are not conclusive, but the case is reasonably strong. Ardipiethicus, or Ardy as some of us refer to him, was probably a fully bipedal ape. An artists rendition made the cover of Time Magazine. Ardy is a celeb.
We don't know how much we're seeing and how much we're missing when trying to extrapolate early hominid ancestry using those few specimens. But odds are it's just a thin slice of a very bushy, hominid evolutionary tree. We don't know how much sexual dimorphism played a role, how social they were, or how they were organized. But it's a fair bet they lived in groups and were organized around kin-bonded communities of related females protected by either an alpha male or a gang of CO-alphas. The Pliocene was full of peril, nightmarish megapredators the likes of which our world has not seen in millions of years stalked mammoth and man alike. Going solo, on the forest floor, out on the Savannas, or furtively foraging between the two extremes, would be risky for a relatively small hominid.
For the better part of four million years our short walk through time has provided us the barest glimpse of only a scattered cache of teeth, bone scraps, and skull bits, imagination and analysis must fill in the yawning blanks. Throughout that span the biggest problem faced by modern day paleoanthropologists was that they didn't have enough material and what they did have was sparse indeed. Then, from over three million years ago a middle-aged little lady would come to light from the hot red plains of Hadar.
Picture the scene in 1974: Afar, Ethiopia. One hundred and ten in the shade-and there is no shade. Only red desert rock and dust baking under the tropical sun. A jeep comes skidding into a remote base camp, the horn blaring wildly, and a young paleoanthroplogist jumped out grinning and screaming as his comrades came running to see what the fuss was, "We've got it! We've got it! We've got the WHOLE bleepin thing!"
AL-288-1, more commonly known as "Lucy", representing a new, previously unknown, small hominid, would burst on the world scene and take the anthropology community by storm. But that is a story for another day ...
