The Most Important Moments for Life on Earth


With the incredible diversity of life found on Earth now, it’s easy to forget that all of it is one big coincidence. Most of us imagine the path of evolution to be straightforward, though it’s anything but. Many random-yet-pivotal events had to happen at the right time in our collective history to make way for the countless life forms we see around us today. Without any of those events, the natural landscape of the planet would be very different from what it is today. 

8. The Last Universal Common Ancestor

Even if there are virtually countless forms of life on Earth today, every one of them shares a single, common ancestor. We know this with certainty due to commonalities found in all the three major branches of life – bacteria, archaea and eukaryotes (which includes all animals, plants and fungi) – though we’re still far from knowing exactly what that creature was. 

Traditional evolutionary wisdom says that this organism – also known as the ‘last universal common ancestor’ – couldn’t have been more complex than the simplest microbes found on Earth today. According to some recent research, though, the LUCA was likely to have been far more complex than we previously thought, even more so than some microbes we know of. One probable explanation is that they initially started out as more complex beings, though lost some of those complexities on the way, in order to adapt to their environments as they branched into their own species.

7. The Cambrian Explosion

It’s difficult to imagine right now, but complex, multicellular life is a rather recent phenomenon in the Earth’s history. For the most part, life existed as simple, single-celled organisms bearing little to no resemblance to any life form today. Most of them couldn’t move and didn’t even require oxygen to survive, and could best be described as barely-alive blobs of living material. That was the case for billions of years after life first emerged on the planet, though it all changed with a massive – and mysterious – event around 540 million years ago.

Known as the Cambrian Explosion, this was when complex, multicellular creatures exploded in population across the oceans, though we still don’t know the precise circumstances that led up to it. The fossils from before that time are conspicuously devoid of any complex life, though something happened around 540 million years ago that caused life to leave its single-celled, kind of pathetic existence, and transform into the slew of oxygen-based, complex creatures we see around us today. 

While we may never know what happened around that time for sure – as archaeological evidence from then remains scarce and unreliable – studies suggest that the Cambrian Explosion may have coincided with the sudden rise in oxygen levels in the oceans at the time. 

6. Sexual Reproduction

The evolution of sexual reproduction may sound like an obvious step for any evolving species, because sex is awesome, though its evolutionary benefits aren’t that immediately apparent. Sexual reproduction comes with the inherent cost of the males. Asexual organisms can reliably pass on double the number of genes than sexual ones, as males – approximately one half of the species – don’t reproduce. Science has never been able to come up with why we would develop sex at all, as no number of benefits seem to offset that massive cost to the species. 

Regardless of its mysterious origins, sex remains one of the most important traits life has developed in the course of its evolution. Sex has played an irreplaceable role in shaping up the world through natural selection, possibly driving the evolution of most complex animals we see around, including us. While we can’t say that life would never have evolved to this extent without sex – as asexual beings are perfectly capable of complex evolution, as well – it sure would have looked a lot different than it does now. 

5. The Great Leap Forward

The last hundred thousand years or so have been particularly eventful for the human species, so much so that we forget that for large chunks of our history, absolutely nothing of note was happening. While ancestors of modern humans have existed for over two million years, for about 90% of the time, there were no noticeable innovations in tool-making, art, or language. This was the time other human species competed for resources with our homo sapiens ancestors, including the Neanderthals.

Then, about 60,000 years ago, something changed. Known as the Great Leap Forward, this was the time when we suddenly – but rapidly – started developing new tools, languages, systems of political organization, and art, marking one of the most important points in the course of our evolution. 

As this was also when we first moved out of Africa to populate Eurasia, some scientists believe that the Great Leap Forward was a result of a widespread drought in Africa, which forced us to look for greener pastures elsewhere. It’s plausible, as we know for a fact that the Earth was going through a massive ice age around this time.

4. The Great Dying

It’s not breaking news that the Earth’s landscape was once dominated by dinosaurs and other large reptiles. The oceans were filled with even more types of humongous and dangerous creatures, making the whole planet a thriving, if deadly, place. 

Come to think of it, the conditions at that time don’t sound conducive for any species other than even larger reptiles, or even larger marine animals, let alone mammals – a relatively harmless branch of life compared to those behemoths. How did we ever survive that?

The answer is the mass extinction – perhaps the biggest one ever – colloquially known as The Great Dying, which happened at the end of the Permian period around 252 million years ago. The mass extinction (which we still don’t know much about) killed off over 90% and 70% of all marine and land species, respectively. While it was definitely a sad event for the life forms on Earth before that, it was what we needed to evolve. 

Fortunately for us all, the immediate ancestors of all mammals – cynodonts – were one of the few species that survived the extinction, inheriting a largely lifeless and empty surface full of opportunities. The earliest mammals were small and had little existential threat from the giant predators of “before.” As a result, they were able to grow and evolve into all the diverse mammals species we see around us today, including humans. The Great Dying is the reason mammals rule over the Earth’s surface today, instead of flying reptiles.

3. The Emergence of Eukaryotes

The term ‘eukaryotes’ may not mean much to most of us, but for an evolutionary biologist, it’s one of the most important chapters in the story of life on Earth. Just like the last common ancestor was a drastic divergence from life forms that came before it, eukaryotes were a crucial divergence from the other two main branches of life; bacteria and archaea. 

Eukaryotes – one of the three major branches of life, which includes animals, plants and all the other major types of life we see around us – are fundamentally different from bacteria and archaea in every way, right from the cellular level. It was an important transition that made many complex functions of our body possible, even if we still don’t know the precise conditions that led to it.

2. When Flowers Took Over the World

Whenever we talk about the biggest factors that contributed to the evolution of life on Earth, we often forget about plants. It’s no exaggeration to say that without the huge variety of plants on Earth – each with their own unique role in maintaining the ecosystem – life as we know it wouldn’t exist. 

What most of us don’t know, though, is that the type of flora we see on Earth today is very different from most of its history. Ferns and cone-producing conifers, not flowering plants, ruled the Earth for the most part. Then, about 130 million years ago, flowering plants came out of nowhere, and rapidly invaded all but the most inhospitable places on the planet. As a result, flowering plants outnumber ferns and conifers by 20 to one today. 

The importance of flowering plants as a crucial step for life on Earth can’t be overstated. Apart from keeping livable ecosystems balanced, flowering plants also provide animals with a source of food and nutrition, without which any evolution – at least for the larger animals – would have been impossible.

1. The Great Oxidation Event

You’d be surprised to know that for about half of Earth’s history, its atmosphere was almost entirely devoid of oxygen. It wasn’t always an oxygen-rich world waiting for oxygen-based organisms to come along and inhabit it. In fact, there’s a good chance that may have never happened at all, if not for something that occurred  around 2.4 billion years ago.

Called the Great Oxidation Event, it was by far the most important event for life on Earth, as well as one of the most mysterious. Around that time, something pumped the Earth’s atmosphere with huge amounts of oxygen, setting the stage for every major development in the course of our evolution since. Some scientists suspect a kind of bacteria – cyanobacteria – that releases oxygen as a byproduct, which may have contributed to the rising oxygen levels at that time. 

Whatever its origins, the Great Oxidation Event was the single most pivotal point for life on Earth, as without oxygen, most of the life forms found on Earth today wouldn’t exist.

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