Episode 5 Field Guide: Throwing in Human Evolution

Humans are weird animals. We walk around on two legs, we have big brains…and we like to throw things at each other. When did all this happen in a gradual march to Homo sapiens? In this episode of Past Time, Adam and Matt talk to Dr. Susan Larson, an expert on the anatomy of living and extinct apes. Dr. Larson’s work shows our close bipedal ancestors had very different shoulder blades than ours that limited their ability to rotate their arms. This arrangement would have made throwing difficult for animals like Homo erectus and you would have wanted to leave him on the bench for the playoffs. The evolution of pitching in recent human ancestors shaped our modern anatomy and is part of what makes us human!

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Click below for illustrations of the animals and anatomy we discussed in this episode and a little more background on the expanding story of your evolutionary history!

Quick Links and definitions:

Anthropology – The word literally means “The study of people.” There are a lot of sub-disciplines in the field. Dr. Susan Larson is a “Physical Anthropologist” because she studies the physical properties of human anatomy and human evolution.

Australopithecus – A genus that contains an ever-fluctuating array of species. They were all bipedal and they all were relatively small, chimp-sized creatures that made simple stone tools in Africa between 4 million and 2 million years ago.

Homo erectus –  A species of bipedal hominin (the name for our direct bipedal ape lineage) that was closer to modern humans in stature and made more sophisticated tools. This was the first species in our lineage to spread out of Africa into Asia.

Shoulder Anatomy – The three main bones of your shoulder joint are the scapula, also called the “shoulder blade”, which is on your back, the clavicle or “collar bone” and the humerus, or upper arm bone. These three work together to support your arm, and are in turn only supported by muscle and the small point of contact between the collar bone and your breast bone (or sternum).

Recent Study on pitching – We discuss a recent study that demonstrated a new theory of energy storage in pitching and its implications for the evolution of our genus. Here’s an article on the study and a video of chimps and humans throwing.

Dr. Susan Larson: Shoulder Evolution Expert

In order to convince Adam that human evolution is as dynamic and scientifically exciting as dinosaur or whale evolution, we contacted Dr. Susan Larson, Professor of Anatomical Sciences at Stony Brook University, to tell us about her work on our own natural history.

Dr. Larson is a Physical Anthropologist which means she studies the evolution of the human body from experimental and paleontological perspectives. Other anthropologists study human artifacts (Archaeology), the diversity of human cultures (Cultural Anthropology), and the genetic diversity of humans (Biological Anthropology).

Dr. Larson starts her work by examining the anatomy of our closest primate relatives. She has had the opportunity to examine the muscles and their function in lemurs, monkeys, humans, orangutans, and chimpanzees. From her observations of anatomy and behavior, she is able to generate models and hypotheses that focus on the steps (pun intended) that brought us from our last-common-ancestor with chimpanzees to our modern form.

One of her conclusions, based on direct observations of fossil apes, is that our  ancestors had a very different shoulder joint than either African apes or living humans. This difference would have allowed them to be tool makers and tool users, but not very effective throwers.

In order to understand where throwing appears in our evolutionary story, we need to understand where we fit in the primate family tree…

Climbing into the Primate Family Tree

We are mammals. We know this because we have hair, drink milk when we’re babies, and have strange ears among many other mammalian features (more on that in future episodes which will make Adam groan). More specifically we belong to a branch in the mammal family tree called Primates. Primates are animals with nails instead of claws, relatively large brains, forward-facing eyes, and short snouts (among many other things, again stay tuned for details).

The main groups within primates are the “prosimians” including the lemurs of Madagascar, lorises (or here), and bush babies from Africa, and tarsiers, small, springy primates from Southeast Asia. Tarsiers are actually at the base of the next great radiation of Primates: The Anthropoids which includes all monkeys and apes.

Here’s a summary family tree:

There are two distinct lineages in the group we call “monkeys.” There are the New World Monkeys that populate Central and South America, a group that includes spider monkeys and capuchins. These primates have flat noses and an extra tooth, while the Old World Monkeys – which includes any monkey from Africa or Asia like baboons and macaques – have larger noses and have the same number of teeth as their close relatives the Apes.

Apes were once a very diverse lineage of primates, but monkeys have diversified in the last couple of million years at the expense of ape diversity and there are only a few species of apes left. We’ll get into ancient ape diversity soon, but for now know there are two kinds of apes in Asia – the small, noisy, acrobatic gibbons (genus Hylobates) and the large, red-haired, and exceedingly endangered orangutans (genus Pongo). Then there are the  “African Apes”: The large, leaf-eating gorillas (genus Gorilla…go figure), the smaller, more omnivorous chimpanzees (genus Pan), and humans (genus Homo).

Humans may be found all over the globe today, but our lineage arose from this African ape lineage with our closest living relatives being the chimps and bonobos (both in the genus Pan). Humans and chimps shared a common ancestor that was moving around in Africa about seven million years ago. Unfortunately we don’t have a fossil record of this common ancestor. In fact, we have almost no information on the independent evolutionary history of chimpanzees and gorillas. The hunt for fossils to fill the seven-million-year gap between the common ancestor of chimps and humans and modern chimp species is one of the great quests of modern paleoanthropology (the study of fossil primates including humans).

However, we have a pretty good idea of what happened on our own evolutionary lineage…

Australopithecus was made for walking

There are teeth and fragments of our earliest fossil relatives from Ethiopia, South Africa, Chad, and Kenya, but the best-known of our earliest relatives belong in the genus Australopithecus.

There are a lot of species that have been placed in – and removed from – the genus Australopithecus and the classification of many specimens remains a hot scientific topic for physical anthropologists, but we can say for certain that this genus was around about 4 million yeas ago and it had already accomplished one of the key traits of modern humans: Australopithecus was built for walking on two legs. The anatomy of the hips and legs in the most complete Australopithecus “Lucy” showed many similarities to our own two-legged arrangement.

After Lucy was found, a series of footprints made in volcanic ash was discovered in Tanzania. Ash is easily dated (more on how geological dating is accomplished will be part of future episodes) and the 3.6 million year old prints from Laetoli were made while Australopithecus was up and cruising the landscape. The prints show three individuals walked across the ash and their feet had big-toes that lined up with the other toes just like ours and a modern arch under the foot, again showing the small-brained Australopithecus was almost modern below the waist (and could have benefitted from Dr. Scholls). But when did we get bigger brains and bigger bodies?

Homo erectus, almost us

One of our closest fossil relatives was Homo erectus, a species that shares our genus it’s so similar to us. They had larger brains than Australopithecus and left behind more complicated stone tools, including the tear-drop-shaped Acheulean hand ax. Despite its name, we don’t know what Homo erectus used the hand ax for, but based on Dr. Larson’s work, we know it probably wasn’t throwing them around…

There aren’t a whole lot of Homo erectus skeletons, but one of the best is nicknamed “Turkana Boy.” The skeleton is from the sediments around Lake Turkana Kenya and Dr. Larson closely examined the anatomy of the Boy’s shoulder and concluded it wasn’t built like modern Homo sapiens…

Shouldering Past

The shoulder is a pretty complicated joint with three main bones: the scapula (shoulder blade), clavicle (collar bone), and humerus (upper arm bone) moving with each other to place your arm and hand wherever you need it. Unlike the hip, there is no solid articulation of the shoulder with the rest of the skeleton except for a small attachment in the middle of your chest where the clavicle and sternum (breast bone) are attached in a mobile joint. The scapula, humerus, and most of the clavicle are anchored to the body by muscle. This is true of all mammals.

Most mammals have their shoulders placed on the side of deep, narrow rib cages – think of the chest of a dog or cat. This is why you can’t get a hug back from a dog or a cat; their shoulders-on-the-side arrangement means they have stability on all fours, but they can’t pull their arms very far out to the side for a loving embrace. But apes have their scapulas placed on their backs. This lets us move our arms out to our sides and even behind us.

Dr. Larson noticed that Homo erectus didn’t quite have its shoulder put together the way we do, or even how a chimp has its shoulder put together. Instead of placing the shoulder joint on the side of the rib cage, parallel to the neck, Homo erectus had a more forward-facing scapula with a forward-placed arm. This meant the transition from a high-shrug-like chimp shoulder to our modern shoulder was not a simple one-step process. Here’s what the shoulders look like in detail…

The forward-facing arm of Homo erectus got the job done for the animal for millions of years, but it was limited compared to our shoulder anatomy. Homo erectus could not pull its arm back. Most detailed tool manufacturing and foraging is pretty straight forward without pulling the arm back very far, but there are a few tasks that can not be done without pulling your arm behind your body. Homo erectus could not throw well, and could not run well.

Throwing and running both involve swinging your arm way behind your body. Check out this slow motion video of San Francisco Giants pitcher Tim “The Freak” Lincecum. Notice how far behind his body he flings his arm to throw the ball.

Tim Lincecum is a professional athlete (with a Red Bull endorsement deal) but even when you throw an object (we’re assuming you’re not a professional pitcher), your shoulder rotates into a position that would have been impossible for our close ancestors. The simple act of running also involves swinging your arm back behind your torso to stay balanced, so throwing and running both would have looked a little odd in Homo erectus.

Precise targeting and accurate throwing may have opened up new opportunities to our direct, pitching ancestors who used newly evolved shoulder-rotating to access new resources and possibly expand the spatial capacity of our ever-evolving brains.

While we may see the implications of throwing in human evolution, the anatomical basis for the movement is still being actively researched. Here’s a link to the study we discussed on the elastic storage of potential energy during a throw. There’s also a video of more pitchers and chimps trying to accurately throw, too.

So, if you want to feel truly human, maybe it’s time to get our a mit and a glove or go for a jog! When you get back, start planning your trip to find the last-common-ancestor of chimps and humans so we can figure out each step of the complicated evolutionary story locked in our shoulder joint.

Thanks for listening and reading. If you have any questions, e-mail us, comment on this post, or post to our facebook page!

For further general reading on human evolution:

The Third Chimpanzee by Jared Diamond (2006)

Evolution: The Human Story by Alice Roberts (2011)

Lucy’s Legacy: The Quest for Human Origins by Don Johanson and Kate Wong (2011)

For more technical information on human evolution:

Primate Adaptation and Evolution by John Fleagle (Third Edition, 2013)

Human Evolution: An Illustrated Introduction by Roger Lewin (2004)

Larson, Susan G. (2007) Evolutionary Transformation of the hominin shoulder. Evolutionary Anthropology 16: 172-187.

Roach, N.T., M. Venkadesan, M. J. Rainbow, and D. E. Lieberman. (2013) Elastic energy storage int he shoulder and the evolution of high-speed throwing in Homo. Nature 498: 483-486. doi:10.1038/nature12267

Filed under: Homo erectus, baseball, evolution, human

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