Stefan Siebert and Charles David

The firing of every neuron in an animal’s body has been recorded, live. The breakthrough in imaging the nervous system of a hydra – a tiny, transparent creature related to jellyfish – as it twitches and moves has provided insights into how such simple animals control their behaviour.

Similar techniques might one day help us get a deeper understanding of how our own brains work. “This could be important not just for the human brain but for neuroscience in general,” says Rafael Yuste at Columbia University in New York City.

Instead of a brain, hydra have the most basic nervous system in nature, a nerve net in which neurons spread throughout its body. Even so, researchers still know almost nothing about how the hydra’s few thousand neurons interact to create behaviour.


To find out, Yuste and colleague Christophe Dupre genetically modified hydra so that their neurons glowed in the presence of calcium. Since calcium ions rise in concentration when neurons are active and fire a signal, Yuste and Dupre were able to relate behaviour to activity in glowing circuits of neurons.

For example, a circuit that seems to be involved in digestion in the hydra’s stomach-like cavity became active whenever the animal opened its mouth to feed. This circuit may be an ancestor of our gut nervous system, the pair suggest.

Neural code

A second circuit fires when the hydra contracts its body into a ball to hide from predators. A third seems to sense light and may help let the animal know when to eat – despite being blind, hydra need light to hunt and they do more of this in the morning.

The team found that no neuron was a member of more than one circuit. This suggests the animal has evolved distinct networks for each reflex – a primitive arrangement, much less complex than our own interconnected nervous systems.

Nevertheless, the hydra is the first step towards breaking the neural code – the way that neural activity determines behaviour, says Yuste. “Hydra have the simplest ‘brain’ in the history of the earth, so we might have a shot at understanding those first and then applying those lessons to more complicated brains,” he says.

Yuste hopes that seeing how the circuits work in real time might lead to new insights into the human brain and tell us more about mental illnesses such as schizophrenia, for example. “We cannot cure patients until we know how the system works,” he says.

Yuste was one of several neuroscientists, including George Church at Harvard University, who launched the Brain Activity Map Project in 2012. It was a rallying cry to neuroscientists, calling on them to record the activity of every neuron in the human brain. The project forms the central plank of the billion-dollar BRAIN Initiative launched by President Obama’s administration in 2013.

Aha moment

The hydra is now the first animal to have one of these maps created for the whole body, although the activity of the whole brains of zebrafish have also been mapped in a similar way. The work is an “awesome milestone worth celebrating”, says Church. But scaling this up to rodents or primates will be very challenging, he says.

Dale Purves, a neuroscientist at the Duke Institute for Brain Sciences, North Carolina, doubts if the animal will prove useful for understanding ourselves. “You have to ask: is this an animal that’s going to join the fruit fly, worm and mouse as a model organism to look at in the quest to better understand the nervous system?” he says. “My answer would unfortunately be no.”

But Yuste is now collaborating with seven other teams to decipher the hydra’s neural code. They want to get such a complete understanding of the way its neurons fire that they can use a computational model to predict its behaviour just from its neural activity.

“One of our dreams is to get to the point in neuroscience that genetics got to when they figured out the DNA double helix,” says Yuste. While some have suggested that the brain is too complicated for that, Yuste is optimistic. “I hope it will happen in our lifetime and it will be an aha moment when the jigsaw puzzle comes together,” he says.

Journal reference: Current Biology, DOI: 10.1016/j.cub.2017.02.049

Read more: “A brief history of the brain”

Our brains followed a twisting path of development through creatures that swam, crawled and walked the earth long before we did. Here are a few of these animals, and how they helped make us what we are.

Hydra



Our single-celled ancestors had sophisticated machinery for sensing and responding to the environment. Once the first multicellular animals arose, this machinery was adapted for cell-to-cell communication. Specialised cells that could carry messages using electrical impulses and chemical signals – the first nerve cells – arose very early on.



The first neurons were probably connected in a diffuse network across the body of a creature like this hydra. This kind of structure, known as a nerve net, can still be seen in the quivering bodies of jellyfish and sea anemones. Urbilaterian



When groups of neurons began to cluster together, information could be processed rather than merely relayed, enabling animals to move and respond to the environment in ever more sophisticated ways. The most specialised groups of neurons – the first brain-like structure – developed near the mouth and primitive eyes.



According to many biologists, this happened in a worm-like creature known as the urbilaterian, the ancestor of most living animals including vertebrates, molluscs and insects. Lamprey brain



More specialised brain regions arose in early fish, some of which resembled the living lampreys. Their more active, swimming lifestyle led to brain-building pressure to mate, find food and avoid predators.



Many of these core structures are still found in our brains: the optic tectum, involved in tracking moving objects with the eyes; the amygdala, which helps us respond to fearful situations; parts of the limbic system, which gives us feelings of reward and helps to lay down memories; and the basal ganglia, which control patterns of movements. Amphibian brain



At some point between the first amphibians moving onto dry land and the evolution of mammals, the neocortex arose – extra layers of neural tissue on the surface of the brain. This part of the brain later expanded hugely, and is responsible for the complexity and flexibility of the mammals – including us.



But how and when the neocortex first evolved remains a mystery. We can't see an equivalent brain structure in living amphibians, and fossils don't help much either: the brains of amphibians and reptiles do not fill their entire skull cavity, so the remains of these animals tell us little about the shape of their brains. Primitive mammal brain



Mammals' brains grew ever bigger relative to their bodies as they struggled to survive in a world dominated by dinosaurs.



CT scans of fossil mammals similar to shrews have revealed that the first region to get pumped up was the olfactory bulb, suggesting that mammals depended heavily on their sense of smell. The regions of the neocortex that map tactile sensations – probably the ruffling of hair in particular – also got a big boost, which suggests the sense of touch was vital too. These findings fit in beautifully with the idea that the first mammals adopted a nocturnal lifestyle to help them dodge dinosaurs. Chimpanzee brain



After the demise of the dinosaurs, the ancestors of primates took to the trees. Chasing insects around trees required good vision, which led the visual part of the neocortex to expand. The biggest mental challenge for primates, however, may have been keeping track of their social lives, which might explain the enormous expansion of the frontal regions of the primate neocortex.



These frontal regions also became better connected, both within themselves, and to other parts of the brain that deal with sensory input and motor control. This all equipped primates to handle more incoming information and come up with smarter ways to act on it. One line of primates, the great apes, became particularly brainy. Human brain



Researchers used to think that taking to two legs caused the size of human brains to outstrip our primate cousins the orang-utans, gorillas and chimpanzees. However, fossil discoveries show that millions of years after early hominids became bipedal, they still had small brains.



It was only round 2.5 million years ago that our brains began to get bigger. We still don't know why, but it's possible that a mutation weakened our forbears' jaw muscles and allowed our skulls to expand.



Once we got smart enough to develop tools and find a richer diet, a positive feedback effect may have kicked in, leading to further brain expansion. Plenty of nutrients are essential for a big brain, and smart animals have a better chance of finding them.



The overall picture is one of an ever-expanding brain, thanks to the interplay between diet, culture, technology, language and genes. That's what brought the modern human brain into existence in Africa by about 200,000 years ago. However, in the past 15,000 years, the average size of the human brain relative to our body has shrunk by 3 or 4 per cent.



To find out why, and read more about the evolutionary journey of the brain, read "A brief history of the brain".