See related note
Photosynthesis (3.5 bln)
"Using oxygen to burn carbohydrates for energy is 18 times as efficient as doing it without oxygen ... Photosynthesis has literally altered the planet’s face, transforming the atmosphere and cocooning Earth in a protective shield against lethal radiation ... This protective umbrella allowed life to escape from the sanctuary of the ocean and colonise dry land ... Life on Earth became high-powered at this point, setting the scene for the development of complex, multicellular life forms"
Without photosynthesis, there would be little oxygen in the atmosphere, and no plants or animals – just microbes scratching a meagre existence from a primordial soup of minerals and carbon dioxide. It freed life from these constraints and the oxygen it generated set the stage for the emergence of complex life.
Before photosynthesis, life consisted of single-celled microbes whose sources of energy were chemicals such as sulphur, iron and methane. Then, around 3.5 billion years ago, or perhaps earlier, a group of microbes developed the ability to capture energy from sunlight to help make the carbohydrates they needed for growth and fuel.
That was a pretty important discovery too: using oxygen to burn carbohydrates for energy is 18 times as efficient as doing it without oxygen.
Life on Earth became high-powered at this point, setting the scene for the development of complex, multicellular life forms – including plants, which “borrowed” their photosynthetic apparatus from photosynthetic bacteria called cyanobacteria. Today, directly or indirectly, photosynthesis produces virtually all of the energy used by life on Earth.
As well as providing an efficient means to burn fuel, oxygen made by photosynthesis helps protect life. Earth is under constant bombardment from lethal UV radiation streaming out from the sun. A by-product of our oxygenated atmosphere is a layer of ozone extending 20 to 60 kilometres above Earth’s surface, which filters out most of the harmful UV. This protective umbrella allowed life to escape from the sanctuary of the ocean and colonise dry land.
“It’s true that many species, including insects, lizards and plants, do fine without sex, at least for a while. But they are vastly outnumbered by sexual ones … Sex may even be responsible for keeping life itself going: species that give it up almost always go extinct within a few hundred generations … mutations are what eventually snuffs out most asexual species”
Origin: billion years ago within ancestral single-celled eukaryotes. Reason: sex creates variation among offspring, helps in the spread of advantageous traits and removal of disadvantageous traits
Brain (580 mln – nervous system in Cnidarians, 270 mln – central nervous system or proto brain in Flatworms)
“The simplest nervous systems are just ring-like circuits in cnidarians – the jellyfish, urchins and anemones. These might not be terribly smart, but they can still find the things they need and interact with the world in a far more sophisticated way than plants manage.The next evolutionary step, which probably happened in flatworms in the Cambrian, was to add some sort of control system to give the movements more purpose. This sort of primitive brain is simply a bit of extra wiring that helps organise the networks.”
"Organisms need to sort out nutritious from toxic food, and the brain helps them do that. Sure enough, look at any animal and you will find the brain is always near the mouth. In some of the most primitive invertebrates, the oesophagus actually passes right through the brain.”
Learn more about the development of the nervous systems
Eye (543 mln)
“And what a difference it made. In the sightless world of the early Cambrian, vision was tantamount to a super-power. Trilobites’ eyes allowed them to become the first active predators, able to seek out and chase down food like no animal before them. And, unsurprisingly, their prey counter-evolved. Just a few million years later, eyes were commonplace and animals were more active, bristling with defensive armour. This burst of evolutionary innovation is what we now know as the Cambrian explosion.”