What Makes Us Human.

Presented by Alice Roberts, this programme follows her journey from five months into her second pregnancy. As an anatomist and doctor, she has a rather different perspective on her pregnancy than most mothers. Before her child is born, Roberts sets out to discover what it is about our bodies, our genes and brains that truly makes us human. We share 99% of our DNA with chimpanzees and yet from the moment of birth, our lives are completely different. At the Max Planck Institute at Leipzig Zoo, Roberts considers these differences, starting with our bodies. Larger brains and upright walking are the hallmarks of humanity. Yet humans are more closely related to chimpanzees than either species is to gorillas. So what has changed in the six million years since we shared a common ancestor? Michael Tomasello is conducting ground-breaking research at the Institute to compare intelligence in chimps and humans. He debunks the myth that there is a ladder of intelligence in the animal kingdom from low to high. Apes are particularly good at understanding spatial and causal relationships. But at the heart of human intelligence, Roberts suggests, is our culture, which depends upon our ability to cooperate. Tomasello demonstrates an experiment showing that while chimps will cooperate, they only do so for selfish ends. A similar experiment, conducted on young children, shows that when humans cooperate they tend to share rewards out equally. Undergoing an ultrasound scan at six months, Roberts discovers that her baby’s brain is already half the size of an adult chimpanzee’s. But the enormity of our brains, coupled with upright walking, makes birth difficult for humans. The human female pelvis is only a centimetre larger than the baby’s head. For decades it was assumed that these two factors – our narrow hips and large brains – led to human babies being born helpless, at a relatively early stage of gestation. But recent research at a New York laboratory has tested the assumptions behind the so-called obstetric dilemma. By observing male and female subjects walking and running, Holly Dunsworth’s team concluded that having a pelvis adapted for birth had no impact on the efficiency of bipedalism. Instead, as her colleague Herman Pontzer explains, gestation is constrained by the human metabolism – beyond nine months, a mother can no longer provide enough energy for the foetus via the placenta. Babies may be born helpless, but their brains continue to develop long after birth in response to interaction with other humans. Roberts dissects an adult human brain, which contains over 100 billion neurons. But it’s not merely the number of cells, but rather the connections between them that make human brains unique. Roberts travels back to the US to learn more about the latest research on the human genome. We have now sequenced the genomes of many other species in addition to humans and can make comparisons between them. Professor Franck Polleux explains that by identifying the differences at base pair level between us and chimpanzees, we are getting closer to knowing what makes us human. A particular gene that affects the developing brain, SRGAP2, is duplicated four times in humans but not in any other species. Experiments conducted on mice show that the gene has a massive effect on the connectivity of neurons. As a result, human neurons form 40-50% more connections than they do in chimpanzees and other animals. At Harvard University, Roberts meets Jeff Lichtman, a neuroscientist working to map the connections in the mouse brain by scanning very thin slices of tissue with an electron microscope. With current technology, it would take 4,000 years to fully image a mouse brain and four million years to image a human brain. But the hope is that by studying these connections we will better understand the how far the development of the human brain is shaped by experience as opposed to genetics.