How did life begin? A simple chronology of life for the last billion years!

How did life begin?

I was inspired by a video posted online [1] that accompanies a new paper showing tectonic plate movement over the last billion years. It’s always interesting to ask ‘how did life begin?’ I thought it would be fun to look at the evolution of animal life as it’s often hard to comprehend. Evolution isn’t a steady continuum. Ever since the very first multicellular lifeforms appeared, it has been accelerating, and is punctuated by periods of immense disruptionThe result of an action that creates a sudden change in the stability of an ecosystem or process. This tends to create a gap where there is free surplus energy and organisms will move in to fill the space. Disruption might be a tree fall, or the application of pesticide to farmland. Disruption is important to maintain dynamics in ecosystems More. It’s a reminder, that animals have been instrumental in controlling the disturbance caused by photosynthesising algae and plants. We owe a lot to animals. Without them, the plants would take over and we’d not have a habitable Earth.

History of life on Earth

A billion years ago

1,000 mya A soup of cyanobacteriaAn early form of life that still exist today and are super-abundant in our oceans. They are similar to bacteria and are not algae but are capable of photosynthesis. More develop natural sunscreen against the Sun’s blistering UV radiation. There’s scarcely any oxygen and no ozone layer yet. This enables them to reach the surface and start photosynthesising. They begin to saturate the ocean with oxygen. This leaks into the atmosphere and starts combining with methane (a powerful greenhouse gas).

750 mya Snowball Earth happens … stripped of methane, the planet is plunged into an ice-ageA period of reduction in Earth’s temperature of between about 4–7 degrees that resulted in the rapid expansion of ice sheets and glaciation of much of the Earth’s surface.  More that lasts 120 million years. This massively accelerates the rate of evolution. This means bacteria can form symbiotic relationships and become multicellular, then under the cold conditions, speciation develops fast.

650 mya The first bilaterally symmetrical animals (Ediacaran fauna) appear in the oceans. This heralds the dawn of a new age of marine animals that diversify to all shapes and sizes. Coral reefs colonies almost all the oceans.

Half a billion years ago

470 mya The first plants start to colonise land. They strip carbon dioxide from the atmosphere, weather rocks, create soil and pollute the ocean with these chemicals. This action starts to literally starve ocean life of oxygen.

375 mya The Devonian mass extinctionAnimal life hasn't existed for very long on planet Earth. In the last 500 million years, there have been five mass extinctions, defined as events that wiped out at least 75% of animal life. The Devonian mass extinction is considered to have been caused by the rise of plants on land, which polluted the oceans in the absence of animals. More happens. The waste from planet-eating plants manage to kill three-quarter of animal life, all of which is currently in the ocean. Ediacaran fauna is wiped from existence.

350 mya Animal life begins re-evolving into new forms and starts to colonise the land. Finally they get control of the planet-killing plants and beginning to stabilise the coupled ocean-atmosphere climate systems.

230 mya The first dinosaurs appear. The warm, oxygen-rich climate suits them so well, they spread out among the adjoined continents. Large animals dominate Earth’s ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More for another 165 million years .

A hundred million years ago

66 mya a damn meteor hits Earth killing most life on Earth and wiping out most dinosaurs for good . It takes another 10 million years or so before the planet’s wildlife starts to recover.

30 mya life reforms in its new design of hairy mammals and feathered dinosaurs (birds). It takes about 25 million years, for ecosystems to reach full capacity again.

10 mya animals have stabilised the climate. They have created present-day ecosystems, sufficiently diverse to support the evolution of even more complex life forms. The temperature of the Earth, the stability of climate and predictable abundance of food, enables the evolution of sophisticated primates. Orangutans, gorillas and chimpanzees evolve.

A million years ago

2 mya the first ancestors of modern humans evolved and began killing all the megafaunaThe largest animals that represent the top of the trophic pyramid. These are the final building blocks in ecosystem structures for maximum entropy production. Megafauna can be measured at any spatial scale. The largest animal that ever lived on Earth is the Blue Whale. In a grassland, spiders could be considered megafauna The term is generally reserved for animals larger More.

300,000 ya Earth finally becomes suitable for modern humans to be the next most likely species to evolve and survive.

The next chapter of life on Earth will be ours … the story is to be continued.

Reference (for the plate tectonic map):

1. Merdith et al. (2021) Extending full-plate tectonic models into deep time: Linking the Neoproterozoic and the Phanerozoic. Earth-Science Reviews, Volume 214, March 2021, 103477. https://www.sciencedirect.com/science/article/abs/pii/S0012825220305237