The Big Bang, The Formation of Earth, and The Birth of Life

This is a pathway about the history of everything that has ever happened. Well, not everything. Consider this a highlights reel, examining the main paradigm-shifting moments in the story of how the universe got from the beginning of space and time to the present day.

It’s a biased history – biased towards one tiny speck in the universe, Earth, and one particular species of primate, Homo sapiens, that calls that tiny speck home. As time goes by, we’ll zoom in in greater detail – the first tile covers over 13 billion years, whereas the last one covers about a century.

But before we get ahead of ourselves, let’s begin at the beginning. Our best explanation for this is the Big Bang Theory.

About 13.8 billion years ago (give or take a few hundred million), the entire universe was squeezed into a singularity. This is a concept so abstract that it isn’t easy to get our heads around. Everything that would ever exist – mass, energy, space, and time – was compressed into a single, tiny, astonishingly dense point.

This point was so dense that time itself was contained within it. Time, in other words, hadn’t begun yet. There was nowhere, and no-when.

Then, suddenly, for reasons that are still up for debate, everything began. This tiny point inflated in a massive release of energy and matter. In the first half a second of the universe, this matter was just particles – a dark, hot soup of electrons, neutrinos and quarks. These then rapidly formed into larger particles – protons and neutrons.

Protons and neutrons collided, forming deuterium, a hydrogen isotope, which fused further, creating helium isotopes, in a process known as nucleosynthesis. At this stage, the universe was too hot for light to shine, as photons were trapped within the dense plasma.

This was the state of affairs for the next 380,000 years – a dark, hot, rapidly expanding mass of subatomic particles, along with hydrogen and helium isotopes.

After nearly 400,000 years of darkness, the next great phase of the universe began. This was what’s known as the ‘recombination’. As the universe cooled, electrons were able to pair with nuclei forming neutral atoms. This set photons free, clearing the cosmic fog and allowing a little light to shine through the universe for the first time.

The cosmic microwave background radiation we can observe today is the afterglow of this event, providing us with a snapshot of the infant universe, approximately 380,000 years post-Big Bang.

So now there were stable atoms, and some light, in the universe. However, it was still extraordinarily hot (about 2700 degrees celsius), and there were no stars or planets. These would come later.

After the recombination, there was a period that lasted several hundred million years, known as the cosmic dark ages. This was the period after the first stable atoms had formed, but before any stars came into existence. It’s called the dark ages because, while there was some light, it was very difficult for it to travel through the dense clouds of hydrogen atoms that filled the universe.

The end of these so-called cosmic dark ages was heralded by the appearance of the first stars and galaxies, sparking the age of re-ionization approximately 400 million years after our universe began.

Around this time, as the universe expanded and cooled, slight irregularities in the density of matter created gravitational wells. Dark matter, making up the bulk of the mass in the universe, began to coalesce in these wells, pulling in regular matter.

The balls of matter that were forming in these wells would become our universe’s first stars and galaxies.

The earliest galaxies were likely small, gradually merging and accreting more gas to form the larger galaxies observed in the later universe. This hierarchical model of galaxy formation suggests that over billions of years, these processes led to the wide array of massive galaxies seen today, each with hundreds of billions of stars.

But how did stars form within these?

In regions of these galaxies where gas was denser, gravitational forces caused the gas to collapse, increasing pressure and temperature until nuclear fusion ignited, giving birth to a star – which is basically a massive, ongoing nuclear furnace.

So, approximately 400 million years after the Big Bang, galaxies and stars began to take shape. This epoch marked a significant shift from a cosmos dominated by simple elements like hydrogen and helium to one enriched with heavier elements.

These complex elements, including carbon and oxygen, were synthesized in the cores of the first stars through nuclear fusion processes. The evolution of stars and galaxies over hundreds of millions of years not only facilitated the creation of these heavier elements but also set the stage for the eventual development of planets and solar systems. This progression was crucial for the diversity of matter observed in the current universe.

The story of our solar system starts about 9 billion years after the universe began. Located inside our Milky Way galaxy, about 25,000 light-years from its center, everything kicked off with a massive cloud of dust and gas called the solar nebula.

This cloud wasn't just any old cloud. It was turbulent and heavy, prepared to become the birthplace of our Sun and the planets around it. Scientists think that a supernova, which is a huge star explosion, sent out a shock wave that made the nebula collapse into itself.

With gravity in the driver's seat, the nebula started spinning and getting hotter and denser in the middle, setting the stage for the Sun to light up.

As the nebula kept collapsing, it spun faster and spread out into a disk. Within this spinning disk, bits of material started sticking together, forming the early building blocks of planets, moons, and other space rocks. Right in the center, where it was super hot and packed, nuclear reactions kicked off and our Sun burst into life, scattering a tremendous amount of energy around and pushing the lighter stuff away from the center—leaving heavier materials to build rocky planets like our Earth.

Further from the Sun, where it was colder, the solar wind didn't blow away the lighter elements, letting gases like hydrogen and helium gather around bigger chunks of rock and ice. This is how the giant, freezing planets, made up of lighter elements, out in the far parts of our solar system, came to be.

Our planet, Earth, formed closer to the sun, accreting from the dust and rocks in the inner solar system, gathering mass until it formed a sphere held together by its own gravity.

The early solar system was a place of great violence. Highly energised protoplanetary bodies, gargantuan meteors, and early planets rattled around like snooker balls, dramatically crashing into one another.

The Earth experienced many cataclysmic impacts from huge bodies hurtling through space, one of which is theorized to have created the Moon. Over millions of years, the leftover debris from the formation of the planets was either integrated into the planets or ejected from the solar system entirely.

After many millions of years of this the cosmic chaos calmed down, and the solar system achieved a more stable state, with the eight planets that we know in our solar system today.

4 billion years ago, Earth emerged from the cosmic dust of the solar nebula. At this early stage, Earth was a brutal place, with searing heat and no protective ozone layer. It was a world devoid of free oxygen, making it unwelcoming to most life forms we recognize today.

However, even in this inhospitable environment, life was already starting to emerge. The earliest life form that we can be certain of came in a form of anaerobic bacteria called cyanobacteria, which we know existed at least 3.5 billion years ago. These probably originated in geothermal springs underwater.

The cyanobacteria grew in layers, binding underwater sediment together into rocky structures known as stromatolites, which you can still see today.

These humble cyanobacteria constructed great stone towers in this way - layering millimetre by millimetre to cover swathes of the ocean with their stromatolites. They were the earth’s first life form. And most importantly, they photosynthesized.

Cyanobacteria were the first organisms to photosynthesize – meaning they harnessed sunlight to extract sugars from carbon dioxide and hydrogen (at this point, it wasn’t from water), creating energy and oxygen as a result. It’s that last product – oxygen – that’s the really important part. Remember that at this phase of our planet’s history, there was very little atmospheric oxygen.

It was thanks to the cyanobacteria, and their millions of years of photosynthesizing, that the atmosphere became gradually filled with oxygen.

This proliferation of oxygen had two major effects in stabilising the atmosphere. The first is that it led to the development of the ozone layer, which protected the planet from harmful solar radiation. The other is that it set the stage for the development of more complex aerobic life forms, who would use oxygen to respire.

There were other dramatic changes that shaped the earth’s atmosphere and made the way for life to evolve. Volcanic eruptions played a huge role in this. These not only spewed lava but also released water vapor, nitrogen, carbon dioxide, and hydrogen into the air.

Together these gases created a greenhouse effect – trapping heat from the sun in our atmosphere and making the planet’s environment both warmer and more stable.

Early Earth also faced relentless assaults from meteorite bombardments. As we’ve discussed, some of these meteorites were large enough to tear off chunks of the earth, one of which became the moon. Others were dozens or even hundreds of miles wide.

The impacts of these early meteorites had a profound effect on the earth’s atmosphere. Often they would send huge amounts of carbon dioxide into the atmosphere, causing major heating of the planet due to the greenhouse effect. This could also lead to acid rain falling around the planet due to increased sulphur dioxide in the atmosphere.

It’s also likely that many of the elements that would later become necessary for complex life, such as carbon and nitrogen, only entered our earth’s atmosphere through meteorites.

So, over several billion years, the earth’s atmosphere stabilised, and, thanks to anaerobic bacteria, became oxygenated. By about 2 billion years ago, the stage was set for more complex life to develop.

For billions of years, life was predominantly single-celled. Microbes such as bacteria and archaea ruled, forming the most complex structures observed in the natural world at that time—colonies like stromatolites.

However, at some point, about 2.7 billion years ago, a major shift occurred that allowed more complex, multicellular organisms to exist. This leap to complexity resulted from a singular, unlikely event: the emergence of eukaryotic cells from simpler prokaryotes.

Essentially, this was the transition from a much simpler cell structure (prokaryotes) to a more complex one (eukaryotic cells). If you learned about ‘cell structures’ at school, it will likely have been a eukaryotic cell. We won’t go into more detail than that, though you can see the difference in the diagram.

The first eukaryotic organisms were single-celled amoeba. These were still ultimately very simple, single-celled life forms. But comparing them to the prokaryotic organisms that came before would be like comparing Albert Einstein to a chihuaha.

These little organisms were complex, autonomous, and much larger than their prokaryotic ancestors. They were capable of moving around and consuming bacteria. This was a massive leap forward from prokaryotes, who would just passively sit around and photosynthesize.

The other hugely important thing about eukaryotic cells is that they allowed multicellular life to form.

The date of the first multicellular organism is widely contested. Some scientists argue that these organisms were in existence as much as 2 billion years ago.

However, the first undisputed multicellular organism can be dated much more recently, to 600 million years ago. This is in the form of Grypiana spiralis, a form of algae. This humble organism opened the flood gates to a whole new world of life forms.

But it wouldn’t be until 60 million years later that the ball really got rolling on complex life forms. This would come with the Cambrian explosion.

The Cambrian Explosion, occurring approximately 540 million years ago, marked a period of rapid diversification in animal life, where most of the major groups of animals first appeared in the fossil record.

This era witnessed the emergence of body plans and physical structures that have persisted to this day, from the hard exoskeletons of arthropods to the spine-supporting notochord of vertebrates.

Predation played a critical role in driving the evolution of these varied body plans, as the need to evade or capture other organisms spurred innovations in mobility, sensory mechanisms, and defensive structures. The ecological arms race during the Cambrian period fostered an explosion of life forms, each adapting to its niche in a rapidly evolving world.

As the Earth's atmosphere and oceans changed, paving the way for complex life, early plants began to colonize land, followed by the first terrestrial animals, including amphibians that evolved from fish-like ancestors adapted to shallow waters. The transition onto land required significant adaptations, such as lungs in place of gills and limbs capable of supporting body weight in gravity.

These early terrestrial ecosystems set the stage for further evolutionary leaps, leading to the rise of dinosaurs, which dominated the planet for millions of years before their sudden demise around 65 million years ago.

After the dinosaurs were wiped out – probably by a meteor impact and the subsequent atmospheric effects – a few, less imposing life forms were left to scavenge the earth. Among these were an odd group of furry creatures known as mammals.

Now that their major predators had been wiped out, mammals were able to develop from small, nighttime creatures into a huge range, exploding into the diverse forms that now occupy almost every possible habitat on Earth.

This age saw the evolution of varied traits such as endothermy (warm-bloodedness), fur, and highly developed brains, setting mammals apart from their reptilian ancestors and paving the way for the rise of primates and eventually, after a few million years, you and me.