created about 1 year ago | Tagged:
Did an autistic inventor start a Stone Age technological revolution? Were the first spiritual leaders bipolar? A daring new theory makes the case
MY DAD hears voices inside his head. "Tiddles" interrupts conversations with cheerful interjections and the odd profanity. "The Bloodbeast" is more sinister, shouting at dad and instructing him to do inappropriate and socially unacceptable things. For most of his life these voices have plagued him and prevented him from living an ordinary life. I know how debilitating and stigmatising mental illness can be, so I must declare an interest in what follows.
In the industrialised world, roughly 1 person in every 25 has severe mental disorder, and nearly half of us will experience some kind of mental illness during our lives. Many conditions, including schizophrenia and bipolar disorder, as well as developmental conditions like autism, are at least in part inherited from our parents. If they affect people's chance of survival you would expect natural selection to have eliminated them, but instead they persist at high levels.
Some argue that these genes bring benefits - mental illness and genius have a long-standing link - but archaeologist Penny Spikins at the University of York, UK, goes further. She believes that mental illness and conditions such as autism persist at such high levels because in the past they were advantageous to humanity. "I think that part of the reason Homo sapiens were so successful is because they were willing to include people with different minds in their society - people with autism or schizophrenia, for example."
According to Spikins, human tolerance allowed the genes associated with different kinds of brain development and mental illness to flourish, kick-starting a revolution. "At some point our ancestors began to develop very complex emotions such as compassion, gratitude and admiration," she says. "These helped them accept and tolerate people with different minds." By embracing the unique skills and attributes that came with unusual ways of thinking, early humans became more inventive and adaptable, and eventually outcompeted all other hominins, she says.
The archaeological evidence is circumstantial, but new findings in genetics are helping to bolster Spikins's idea. It turns out that some genes associated with mental illness proliferated at just the time when human society was flowering and confer attributes that other hominins may not have shared. All of this raises the interesting issue of whether, in the modern world, we should place more value on people like my father.
Consider stone tool technology. Emerging around 2.6 million years ago, the first stone tools were crude and developments slow. For hundreds of thousands of years our ancestors made do with an array of hand axes, scrapers and thrusting spears. Then, around 100,000 years ago, there was a technological revolution, with many new and sophisticated implements appearing. "We see a sudden change in the archaeological record to more standardised tools that are more precisely made and more technologically focused," says Spikins. The invention of spear throwers, bows and arrows, fishing harpoons, traps and snares, for example, allowed hunters to distance themselves from their prey and so hunt more aggressive animals.
Spikins argues that this technological tool revolution may have been triggered by a greater tolerance for people with traits on the autism spectrum. "I'm not saying that someone who isn't autistic wouldn't understand this technology, but that the innovation is more likely to come from someone who is systematic and has that unique focus on precision," she says. Spikins also notes that other hominins, including Neanderthals, show few signs of tool innovation and never reached the level of sophistication achieved by our ancestors.
At around the same time as the tool revolution, archaeologists see a burst of artistic creativity. This begins with things like shell and bead necklaces and decorations on bone and ochre, and later carved figurines and simple musical instruments. By about 35,000 years ago modern humans were painting stunning lifelike animals and people on cave walls. The striking similarity between these and the drawings produced by some autistic people with savant-like abilities has been noted by Nicholas Humphrey at the London School of Economics (Cambridge Archaeological Journal, vol 8, p 165). "This emergence of very realistic art, which is drawn with incredible precision, could be linked to the tolerance of autistic people, who had those special skills," says Spikins.
Evidence of religion and spirituality also appear during this period. It has been argued that shamans were responsible for painting the more metaphorical and dream-like cave art, and Spikins believes they would have had a big impact on society. "I think that they helped to bind people together, by helping them to make sense of their world, through myths, ritual and a belief in a spirit world," she says. In modern hunter-gatherer societies shamans tend to be unusual and creative people, who sometimes go into trances. Some also express traits associated with schizophrenia, such as hearing voices, and other mental disorders. "Using modern criteria to diagnose mental illness, I think we would say that most shamans appear to have had mood disorders - most probably bipolar disorder," says David Whitley, author of Cave Paintings and the Human Spirit (Prometheus, 2009).
Anthropologist Henry Harpending at the University of Utah in Salt Lake City says Spikins's idea is plausible. "The points she makes about standardised tool types and the detail in art certainly ring true." But would mental disorders have passed the "survival of the fittest" test? "People with bipolar disorder might very well bind communities together, but first they have to survive and reproduce themselves," he says. "In the case of schizophrenia, we have data in contemporary industrial societies and we know it causes a big fitness reduction." Different minds may have contributed to our evolutionary success, says Catriona Pickard from the University of Edinburgh, UK, but mental illness is more likely to be an unfortunate by-product of evolving a highly developed brain (Cambridge Archaeological Journal, vol 21, p 357).
Others argue that modern society is not a good analogue for the past. "Eccentricity is much more accepted in small scale hunter-gatherer societies, as everyone has a role to play," says Benjamin Campbell, an anthropologist at the University of Wisconsin-Milwaukee. He is supportive of Spikins's idea and agrees that complex emotions such as compassion set us apart from other species. "Modern humans understand that someone else has different thoughts, and we have developed this ability to a tremendous extent," he says. For sure, the archaeological evidence is circumstantial, but genetic studies are starting to put the theory on firmer ground. Last year's sequencing of the Neanderthal genome showed that around 99.8 per cent of the genes were the same as those of modern humans (Science, vol 328, p 710). That is hardly surprising, given that we shared an ancestor within the past 500,000 years. But David Reich at Harvard Medical School in Boston points to some key differences. "We found that Neanderthals carried subtly different forms of the AUTS2, CADPS2 and NRG3 genes compared with modern humans," he says. AUTS2 and CADPS2 are associated with autism and NRG3 with schizophrenia. However, Reich adds, it is not clear whether these differences influenced the way our ancestors thought.
Neanderthals became extinct around 30,000 years ago, and Spikins is not alone in thinking that superior adaptability helped early modern humans outcompete them. Whether our ancestors were aided by having genes associated with mental illness is another matter, but the fact is that humans are the only hominin that survives to this day. What's more, in recent years it has become clear that our ancestors had more contemporaries than had previously been thought. Around 80,000 years ago, there were at least five species of hominin roaming the planet, says Chris Stringer from the Natural History Museum in London. While H. sapiens predominated in Africa and Neanderthals were dominant in western Eurasia, descendants of H. erectus probably still survived in Indonesia, H. floresiensis (aka "the Hobbit") resided on the Indonesian island of Flores and the newly discovered Denisovans seem to have ranged across a swathe of Asia (New Scientist, 2 August, p 34). We don't yet know whether early modern humans had more genes associated with mental illness than these other species, but we may not have to wait long to find out, given the speed with which ancient DNA sequencing is progressing.
What we do know is that inherited mental illness is extremely rare among living primates. Klaus-Peter Lesch at the University of Würzburg, Germany, and colleagues looked at the gene responsible for the serotonin transporter protein, SERT, which has been implicated in several inherited disorders. Responsible for regulating the movement of serotonin - a neurotransmitter crucial to mood, among other things - the gene comes in a "long" and "short" form. Every human carries a combination of two of these. People with the long/long combination appear to be protected from very low mood, whereas those with the short/short or short/long variants are more susceptible to depression. Lesch and colleagues looked at the gene in 12 species of primate and found that the short version is found only in humans and one other primate, rhesus monkeys (Molecular Psychiatry, DOI: 10.1038/sj.mp.4001157). "Carrying one of the short variants of the SERT gene seems to expose humans and rhesus monkeys to emotional dysregulation commonly associated with emotional disorders, which we don't see in other species," says Lesch.
But it can also confer advantages. "The short variant appears to be linked with emotional responsivity. In a stressful environment this seems to increase vulnerability to depression, but in a good and nurturing environment people with this variant are often highly successful, with excellent communication and social skills." And the benefits may extend even further. "One trait that humans and rhesus monkeys share is their ability to live almost anywhere," say Lesch. Noting that other primates thrive only in very specific niches, he speculates that behavioural traits connected with the short versions of the gene for SERT may have helped both humans and rhesus monkeys adapt to new and challenging environments.
Such adaptability would have been crucial in the past 50,000 years as our ancestors migrated around the world, and it turns out that the gene responsible for SERT is among many that evolved rapidly during this period (see The 10,000 Year Explosion by Henry Harpending and Gregory Cochran, Basic Books, 2009). The genetic analysis that revealed this dramatic acceleration in human evolution also exposed the rise of another gene variant linked with mental disorder - this time one that helps regulate dopamine, the neurotransmitter associated with pleasure and reward. Harpending and colleagues found that a particular variant of the gene that codes for the D4 dopamine receptor has increased very rapidly in frequency in humans. People with this variant, known as DRD4-7R, tend to have very high energy levels and an increased risk of attention-deficit hyperactivity disorder (ADHD). Yet the prevalence of the variant among certain groups - it is found in 80 per cent of lowland Amazonian Indians, for example - indicates that extra energy has its advantages. "Previously these traits have been highly regarded in some societies," says Lesch.
"We see a higher percentage of ADHD-associated traits in migratory people, for example." Like the SERT gene, DRD4-7R can be both a boon and a bane. Some researchers describe such genes as "orchid genes": nurture them and the carrier thrives, neglect them and a maladaptive personality trait appears. If Spikins is correct, many other genes associated with developmental conditions and mental illness should possess such Jekyll-and-Hyde characteristics. Our ancestors may have benefited from this, but modern societies tend instead to view different minds as a major impediment. "Nowadays, being 'mad' is bad," says Whitley. "In the west, we continue to pathologise difference, and lose its potential adaptive advantage."
Instead of ostracising people with maverick minds, perhaps we would do better to cherish them (see "Beautiful minds?"). My dad, for one, has always maintained that having schizophrenia is beneficial in some ways. If the special talents possessed by some people like him have helped to get us this far, we may need their different ways of thinking to see us through the next few thousand years. If the past teaches us anything, it's that humanity thrives by being adaptable.