BACK in 2012, neuroscientist Christof Koch wrote in his book Consciousness: Confessions of a romantic reductionist that the human brain is 鈥渢he most complex object in the known universe鈥�. Given that there are about 86 billion neurons in a brain, connected up in ways that we are only beginning to unravel, this seems intuitive. But when I put it to at the Santa Fe Institute in New Mexico 鈥� created in the 1980s as a hub for the budding field of complexity science 鈥� he doesn鈥檛 see it that way. 鈥淚t鈥檚 almost farcical to entertain that we are the most complex system in the universe,鈥� he says. 鈥淭he question is actually wrongheaded.鈥�
Despite this, I persevere. Surely, there is some common measure of complexity that can be applied to all kinds of intricate systems? After all, if you squint, clusters of galaxies and the filaments that connect them look like tangled circuits of neurons. There are even roughly the same number of neurons in the human brain as there are galaxies in the observable universe. This similarity in form may have something to do with general laws through which complexity emerges, says at Pompeu Fabra University in Barcelona, Spain. Or it may not. 鈥淐oincidentally, that might appear in both systems, but that doesn鈥檛 mean anything,鈥� he says.
Besides, complexity isn鈥檛 defined by components and their interconnections. It is the idea that the whole is more than the sum of its parts 鈥� something that complexity scientists call emergence. In this way, the human brain is more like an ant colony 鈥� creating elaborate underground cities without a blueprint or a leader 鈥� than a galactic web. Neurons and ants are both relatively simple units that, somehow, collectively give rise to sophisticated behaviour. Sol茅 reckons that the human brain transcended its neurons owing to our unusual capacity for language. 鈥淔rom our lexicon, we can build virtually an infinite array of sentences,鈥� he says. Combined with our memory, this allows us to foresee the future and even imagine alternative futures, as well as the minds of others.
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But language, memory and imagination aren鈥檛 attributes of all complex systems. In 2001, looking for common ground, physicist at the Massachusetts Institute of Technology . First, how hard is it to describe? Second, how hard is it to create? Third, what is its degree of organisation? He went on to list about 40 different measures of complexity that fit into his general framework. 鈥淭here have been many more since,鈥� says Wolpert.
One of these is assembly theory, which determines how evolved any object is by considering how many steps were required to build it, and to what extent this object leads to further complexity. 鈥淗uman brains make very high-complexity objects in abundance, so, by direct extension, they are the most complex object,鈥� says at Arizona State University. Another measure is integrated information theory (IIT), which assesses the amount of consciousness in a system according to how densely interconnected its network is. The human brain is also highly complex by this measure.
Some complexity theorists don鈥檛 buy into assembly theory or IIT, and all agree that these measures fall short of a general definition of complexity. Wolpert goes further, arguing that the value of complexity science doesn鈥檛 come from finding a universal measure, but from the diversity of definitions.
So, where does that leave the question of the human brain? Wolpert is in no doubt. It is pure hubris to see it as the most complex object in the universe, he says. 鈥淣othing concerning humans is ever privileged. At best, it鈥檚 run of the mill 鈥� the brain included.鈥�
