A Brief History of Plant Life on Earth

A Brief History of Plant Life on Earth – 2 Part Series

Life is truly ancient. You, me, the grass and the trees  – we are all inextricably connected to something that happened on a young Earth around 4 billion years ago. Scientists have arrived at a number of theories offering an explanation as to what happened, but there is a general consensus that living organisms arose from simple, naturally occurring organic compounds. Informed  by the current reservoir of scientific theory, the next two segments will attempt to take you back before anything that could be described as a plant existed, to the very beginning of life on Earth. Then, this two part series will race through the slow process of increasing complexity that brought us all here, to the moment when you are reading this, thinking about where the plants dominating our landscapes really came from.

 

Part 1 is about the formation of our planet, the origin of life on it and the fascinating processes that gave us cells, photosynthesis, and multi-cellular organisms, played out against a backdrop of violent meteor bombardment, volcanic eruptions, worldwide hurricanes, monster tides and mass extinction events.

Part 2 (coming soon) traces the history of land plants, from the time that pioneering sea plants began to colonise the Earth’s surface to the vast forests, flowering meadows and sprawling grasslands of today.

 


 

Part 1 – The Seed of Life

4.6 billion years ago, our planet did not yet exist. Accretion discs, solar nebulae, and primitive chondrites were the principal characters of our unrecognisably empty solar system. In simple terms: the Earth was being formed from clouds of dust in space collapsing in on themselves, pulled together by gravity, orbiting around the newly formed Sun.

Early on, just as our planet had been fully established, a catastrophic event resulted in something critical to the existence of life: the infant Earth collided with another planetary body, producing a large quantity of rock debris that went into orbit round our planet. Under the force of gravity these rocks were pulled together to form earth’s natural satellite, the Moon.

This time period of Earth’s history is known as the Hadean Eon, named after Hades, Greek god of the underworld. It is an appropriate name – the newly formed Earth was being bombarded by meteors, an ocean of liquid magma covered the planet’s surface, and volcanic eruptions were spewing molten rock and toxic gases, making for a hellish environment.

After a while, the hot Earth began to cool. It was during this time that the oceans were formed – competing theories as to how this happened include water being delivered by icy comets from space, or water locked up in the Earth itself being released by meteor impacts. Either way, the formation of the world’s oceans was essential for the emergence of life. For our purposes, it is enough to know that those ancient waters harboured a small collection or organic molecules, and when those molecules came together, life was born.

At first, life looked like simple, single celled structures, similar to the bacteria we see today. They did very little, except from congregate in masses on the ocean surface, or around blistering hot hydro-thermal vents on the ocean floor.

Gradually, life became more complex. Hurricane force winds swept through the empty landscapes, and giant tides exposed then engulfed shorelines. These forces are thought to have facilitated mixing and interaction between distinct bacteria populations living in the oceans, which lead to some highly interesting developments that would have far reaching repercussions.

One of these developments was photosynthesis, now exhibited by all plants. Today, photosynthesis plays a major role in sustaining life, by contributing to the atmospheric oxygen that allows living things to breathe. Back then however, around 2.4 billion years ago, photosynthesis spelt death for most of the world’s organisms. The slow build up of oxygen in the atmosphere, brought about by newly developed photosynthesising bacteria, was toxic to the other bacterial life that had already evolved in an atmosphere free of oxygenated air. Over time, the never before seen quantities of oxygen poisoned all other life on the planet. The scientific community refers to this as the Great Oxygenation Event, and it was the world’s first extinction.

This was the first extinction event in a long line of mass die-offs that characterised the rest of Earth’s natural history. It is accepted that 99% of all plant and animal species that have ever existed on this planet have gone extinct – that’s a lot of plants we will never enjoy in the garden!

The photosynthesising bacteria slowly gathered together, and over long periods of time developed into colonies of multi-celled mats of algae. These algal mats were among the first organisms to bear resemblance to the plants we see today – they were green, they photosynthesised, and they lacked mobility. These sea-faring proto-plants were the ancestors of all plants alive today.

Plant life, until relatively recently, only existed in water. We shall stop our journey through the evolution of plant life here, at these algal mats floating in the oceans. Part 2 of this series will pick up exactly where we left off, detailing the incomprehensibly challenging task of moving from the oceans to colonise a hostile and unknown environment: land.

 

 

 


 

Igor Knowlson

 

Igor is our resident intellect who writes monthly on his thoughts on plants