Episode 1: First Steps

Paleoanthropologist Zeray Alemseged discovered fossil called “Selam,” also known as “Lucy’s Child.” Lucy and Selam are 3.3 M years old but humans and chimps split around 6 M years ago = first biped. But brains not necessarily bigger as a result. Small-brained bipedal apes were around for 4 million years, flourishing for 25 times longer than we’ve been around. Length of childhood is a key difference between chimps and humans. Chimps almost fully formed by age 3 vs. humans 2 decades for brain to mature. Selam’s brain – 300 cc = 75% of adult size suggesting slower maturation. Lunate sulcus moved back as brains evolved toward modern human brains. About 2.5 million years ago, we find first stone tools. Around 2 M years ago, genus homo appears. Homo Habilus = first tool maker and more dexterous hands. Much smaller than us about 3-4’ tall (like Lucy), but brain size about double the brain volume (700-800 cc).

Why, after millions of years of flat lined brain size, did brain volume increase 2 M years ago? Rapid climate change, including fluctuating lakes and droughts (sometimes within 1000 years), may have been the catalyst for our evolution. Deep sea sediments can help us read millions of years of climate. African climate was stable and dry for 3 million years between Tumai and Salom, followed by 200,000 years of wild fluctuating climate and during this time tools appeared along with larger brains. Rick Potts – climate variability was the driver of human revolution.

Episode 2: Birth of Humanity

First human = Homo Erectus, 1.3-2 M years ago, Rift Valley, Eastern Africa. Basically us. Left Africa. First human societies. Firemakers.

How can we know what these early humans were like? Meave & Richard Leakey working in Kenya in 1984, found “Turkana Boy”, a complete Homo Erectus skull and the earliest and most complete skeleton found. He was 5′ 3″ and about 8 years old & thus had a growth rate more similar to chimps than modern humans. Brain was 900 cc (not as large as ours but twice size of Chimp brains). Modern broca’s areas suggest symbolic language. Stone hand axe marks start of technology. Stone hand axes allowed huge expansion in the varieties of food people could eat.

Mystery of prolonged childhood in humans. Perhaps due to brain size because brain growth needs to occur after birth. Modern brains consume 25% of our energy and thus need more calories. These are most easily met through eating meat. Homo erectus was not at the top of the food chain. How to kill an animal without that animal killing you? Distance running (b/c lost body hair — we probably lost most body hair about 3 M years ago — around Lucy’s time — based on genetic divergence of head lice and pubic lice) and can sweat to cool off unlike most other animals) and high activity during the day. Persistence hunting is therefore possible — run the prey to exhaustion in the  middle of the day, never giving the animal a chance to cool down.

Social complexity: relationships, communication, rules, symbols. Why did we become so social? Richard Wrangham, Harvard University, theory: Due to building fires and cooking? Likely built fires to stay safe at night. Cooking would also explain the evolution of smaller teeth and stomaches bc cost of digestion is significantly lower. We learned to share and communicate waiting around fires for food to cook.

Apes are in almost constant contact with their babies, but human Mothers are not. So, human babies have to be really sensitive to good caregivers and be able to get what they need from multiple caregivers. Part of necessity of caring what others think. Turkana Boy may have died from a very painful tooth absess that infected the rest of his body, a possibility that would imply that he was cared for by others.

Greater cooperation may have been responsible for our exit from Africa. When did they leave Africa and why? First humans to leave Africa were much more primitive than Turkana Boy but used stone tools. Discoveries in Georgia show that humans left Africa about 1.8 M years ago. Now scientists believe that Homo Erectus left Africa as early as they evolved and possibly earlier. Why did they leave? Probably a climate shift that spread grasslands from Africa into Asia and game animals followed and hominids followed the animals. Process was probably very slow.

Homo Erectus probably survived until 50,000 years ago. Our own species has only been around for 200,000 years.

Last Human Standing

At least 20 types of human ancestors have existed between now and 6 M when we split off from apes. 

4 types of humans living simultaneously up to about 50,000 years ago. “Hobbits”=three-foot-high humans who thrived on the Indonesian island of Flores until as recently as 12,000 years ago.

Daniel Lieberman (Harvard University): “Humans have a very intensive way of using the environment. Humans move into the Middle East and the Homo Erectus starts going extinct. When humans move into Europe, the Neanderthals go extinct.”

Why were Homo Erectus and Neanderthals replaced by modern humans? Neanderthals had bigger brains than ours. Neanderthals (lasted more than 400,000 years) likely matured more quickly than we do. They had similar frontal cortices as we do. Neanderthals were almost exclusively meat eaters (and not seafood) and this does not vary by their location. They got close to the game and most males had multiple fractures. Very few lived beyond the age of 30. Neanderthals disappeared about 25,000 years ago.

Homo sapiens evolved 200,000 years ago: Longest, coldest glacial periods on record. Mega-droughts in Africa. This crashed populations at the time. All humans are 99.9% identical vs. apes have much more genetic diversity. Clue to a crisis that may have wiped out huge numbers of people. 140,000 years ago, most of tropical Africa became uninhabitable and we teetered on the edge of extinction. All modern humans are descended from about 600 breeding individuals from Africa (29:55; Curtis Marean, Institute of Human Origins), perhaps on the South African coast, who made it through these crises. We lost a huge amount of genetic diversity.  It was possibly living by the sea that caused big changes in these small numbers of humans — broadening diets (31:24), including shellfish, requiring new and more versatile ways of doing things. To harvest shellfish, you have to predict low tide and knowing moon cycles helps with this. Larger variety of tools proliferated and the tools carry social information (they are more complex than needed to do the job) and decorative art emerges around 75,000 years ago = “storing information outside of the human brain”. As climate improved, we started to stream out of Africa.

What happened when modern humans met Neanderthals? Svante Paavo (Max Planck Institute) have mapped the Neanderthal genome. Comparing human and Neanderthals suggests an identical gene related to ability to speak. Do Neanderthals and humans have a common ancestor? Homo Hedelbergenses (sp?) is that ancestor. No evidence of Neanderthals interbreeding with humans. Probably the density of Neanderthals was low — big body and big brain in cold environment and needed about 5,000 kcals daily. Modern humans had lower energy demands, populated well, and then developed throwing spears, allowing humans to hunt broader range of animals more safely and therefore had greater ecological niche. The arrival of homo sapiens precipitates the extinction of larger species. Last Neanderthal was probably at Rock of Gibraltor.

Culture is the way we adapt. We pass it on to our kids. Genetic and cultural evolution interactions. Rate of genetic evolution has increased over last 10,000 years and likely to continue.