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Brain evolution



The human brain is the most complex phenomena in the known universe.


When you were in your mother’s womb, each of your 100 000 000 000 brain cells knew how to wire up and what to become. This astonishing process continues into your late teens, sculpting the person you are.


However, the truly fundamental factor is not only how this astounding process happens, but why it happens.


Why did the human brain become so complex? Why did we learn to speak? Why do we have certain behaviours? Why did we become so intelligent? And how do our brains differ from a monkey’s or a dolphin’s?


All these questions can be answered by the mind-blowing and truly fundamental theory of evolution.


What is evolution?

Darwin’s theory of evolution proposes that animals well suited to their environment survive - and pass on their genes. Animals that are not well suited perish before they have offspring. Their mixture of genes die with them.



Over the course of millions of years, this has led to an astounding array of different creatures and organisms on our planet.

Each perfectly suited (ie adapted) to it’s own environment. Ant-eaters with long noses to probe ant-hills, sharks stream-lined to speed through water and bees that work together in a hive.

So how did brains evolve?

If you didn’t know about the theory of evolution, how would you explain where brains came from? One option would be they all appeared on the planet one day (the creationist argument).


However, armed with an understanding of evolution, you can look at the world in a new way – and work out how animal bodies and behaviours have given them a survival edge over their competitors.


Our brain cells, brain molecules, neurotransmitters and synapses are almost identical in all animals – so the brains of insects, fish, reptiles, birds and mammals are all made from the same building blocks.


Again evolution can explain the amount of brain devoted to a particular task.

Crocodiles have huge olfactory bulbs, the area of the brain that deals with smell. In contract, humans have vast areas of the brain devoted to vision.


Evolution can even explain how the vast array of animal behaviours came into being.



Early brains

Early brains on our planet were very simple – and are found now in animals lower down the evolutionary ‘tree’ for example in insects, worms and snails. These early brains are more collections of ganglia – where hundreds of nerve cell bodies congregate.


Fish and amphibians have well defined brains – albeit small ones in relation to their body size. Reptiles and bird brains become ever more complex with areas devoted to specific senses, for example vision and smell.


Mammals have a vast variety of brain shapes and sizes. The biggest brain on our planet belongs to the blue whale – weighing in at 6kg, compared to the 1.4 kg brain of a human.


Does size matter?

Interestingly, size isn’t everything – and provides us with a bit of a puzzle.


The North American ruby-throated hummingbird has a brain weighing less than a gram, where as a blue whale has a 6kg brain. Yet both show a marvellous variety of behaviours. Both sing, defend territories, attract mates, raise young and migrate seasonally for long distances. The tiny-brained hummingbird also has an elaborate courtship dance, builds nests and solves some interesting pattern-recognition problems in finding flowers.


Do intelligent mammals have bigger brains?

In general yes, but only when considered as a proportion of their overall body size.



A good indication of intelligence is brain weight in relation to body weight.


Body weight
Brain weight
How much of the animal is its brain?
Blue whale
60 000 kg
200 kg
70 kg
1.3 kg



It seems that carnivores have bigger brains in relation to their body size than their prey – presumably giving them the advantage to create tactics and strategies to catch prey.


Animals that eat seeds and fruits (frugivores) have larger brains than similar sized animals that eat foliage and leaves (folivores) presumably allowing them to distinguish the colours and shapes of ripe fruits to satisfy their discerning tastes.

Careful parents outrank the careless in terms of relative brain size.


And animals with proportionally bigger brains show a wider range of complex behaviours.


When did humans evolve such huge brains?

Hominid brains have evolved and grown from 400g 3-4 million years, to their present size of 1400g (1.4kg).


The bodies of Homo erectus (1.7 million years ago) were not substantially smaller than humans of the last century, yet their brains were nearly half the size.


There are lots of questions about brain evolution that scientists are still working on – but there is overwhelming evidence that brain complexity and ‘intelligence’ are hugely beneficial in an evolutionary sense.


Brain evolution and babies brains

More intelligent mammals such as dolphins, chimps and humans have highly convoluted brains compared to the smooth brains of less intelligent animals.


However, intelligence is also related to how big an animal’s brain is relative to its body size.


Interestingly, as we developed in our mother’s womb our brains also had a smooth surface until 6 months, developing convolutions before birth.


This picture shows three model brains.


The smallest is a premature baby’s brain (at 26 weeks), the middle is a newborn baby’s brain and the largest is an adult’s fully-grown brain.


Premature babies are born with a smooth brain, and the convolutions develop in the few months after birth.


Why has consciousness evolved?

This question continues to puzzle scientists. Why is it that humans have an awareness of their own existence, and why does this give an evolutionary advantage? Perhaps consciousness is needed to make sense of what you see (find out about blind sight).


There is still huge debate over which animals are conscious – click here to tell us what you think.


And finally...

An understanding of evolution is fundamental to understanding brain science – and indeed all biological sciences.


Over the past several hundred years, there have been two, perhaps three fundamental upheavals in human thought.


Firstly, The Copernican Revolution - that far from being the centre of the universe, the earth is simply one of several planets orbiting our sun.


Secondly, The Darwinian Revolution - that by the astounding process of evolution, every living thing on the planet has evolved into the huge variety of colours, shapes, sizes and behaviours we see today.


(One could argue that understanding DNA was the third revolution, and in the future understanding the human brain and consciousness might be the forth).


In some parts of America, there is considerable debate over whether evolution should be taught as an accepted scientific theory in schools.


Click here to take part in our international evolution survey


Click here to see an astounding collection of animal brains


Click here to read more about brain evolution