Despite being a turbulent place, early Earth had all the conditions needed for life to develop. Image © Gregoire Cirade/ Science Photo Library
By Hayley Dunning
As far as we know, Earth is the only planet that supports life. But how did it begin, develop into cells and diversify into the great complexity of different forms we see around us today?
Join us on a four-billion-year journey to discover not only how Earth gave rise to life but how life itself changed the face of the planet.
Earth formed around 4.6 billion years ago from the dust of the early solar system.
Its first half a billion years were chaotic as meteorites bombarded its surface. But eventually things settled down, the oceans formed and the atmosphere stabilised.
Life quickly took advantage of this new calm. The first life was single celled and stayed this way for billions of years. The first evidence of life’s influence is debated, coming potentially as early as 4.1 billion years ago, but the oldest direct evidence – the first uncontested fossils – is about 3.5 billion years old.
Life needed a few things to get going, including important chemical elements, the right location and a good deal of luck.
But life is more than a series of ingredients. Something has to happen to get them together in the right order. The first step is to form amino acids – the building blocks of life.
This rocky material comes from the asteroid Bennu. Our scientists were part of a team that identified some of the building blocks needed for life to develop inside the Bennu samples brought to Earth by the OSIRIS-REx mission.
A famous experiment in 1952 was pivotal in showing how this might have happened. The Miller-Urey experiment recreated the conditions on early Earth and showed how inorganic molecules – basic chemicals such as methane, ammonia, water and hydrogen – could produce amino acids. The crucial ingredient was a spark – an electrical arc simulating lightning.
Ideas about the chemicals available in early Earth’s atmosphere have changed over time, but further experiments have shown similar results – the building blocks for life can assemble from inorganic molecules.
But is this what happened on Earth? Astonishingly, recent research has shown that some asteroids may carry amino acids and water, raising the possibility that they delivered important chemicals to Earth. This could also mean life is abundant throughout the universe.
Charles Darwin pictured a “warm little pond” with all the necessary chemicals and an added dose of light, heat and electricity to bring them together. But ponds lack one key factor – time. They can be quickly created and destroyed, so a steadier environment was needed.
Hydrothermal vents today are teeming with life. Image from NOAA via Wikimedia Commonsopens in a new window, public domain.
Other warm-water environments, such as hot springs and volcanic lakes, are possibilities, but many scientists think hydrothermal vents are the best bet. These deep-ocean structures spew out fluid that’s been superheated beneath Earth’s crust and carries dissolved gases and minerals. In the surrounding freezing water of the deep sea, this fluid cools rapidly, providing conditions around the vents that are just right for life despite the lack of light.
However and wherever it happened, once amino acids formed from inorganic molecules, they began to assemble into simple proteins, setting the stage for self-replication and cells – the hallmarks of life.
Before the first definite fossils, some scientists argue we have evidence for life in chemistry. Living organisms prefer one type of carbon over another and the signature of this ‘fractionation’ may be seen in rocks from Australia and Greenland that are nearly four billion years old.
Other candidates for the earliest signs of life are found in a 3.65-billion-year-old rock formation in Australia called the Apex Chert. It contains tiny filaments – long, thin shapes less than a hundredth of a millimetre long. Powerful microscopes reveal they look like modern cyanobacteria – also known as blue-green algae – and their chemistry suggests the presence of organic molecules. However, some say they could have been made by natural geological processes. The same goes for potential early microfossils from rocks formed around ancient hydrothermal vents.
This wafer-thin slice of rock from Western Australia may contain the world’s oldest fossils – although there’s disagreement around their interpretation.
Stromatolites, the first uncontested fossils, were formed by colonies of bacteria. These structures occur when layers of sediment trapped by microorganisms such as cyanobacteria, build up, resulting in a striped rock with a distinctive structure. They were widespread in early shallow seas. Living stromatolites still form today in areas including the Bahamas and salty lagoons in Australia.
Stromatolite colonies of cyanobacteria used water, carbon dioxide and sunlight to make sugars – essentially an early form of photosynthesis. This produced oxygen – at the time there was only a very low level of this gas in the atmosphere.
Stromatolites were widespread in early shallow seas. They still form today – these are living stromatolites in Shark Bay, Australia. Image © pst.rtw/ Shutterstock
Another striped rock gives us our best clue as to what happened next. Banded iron formations, which are potentially up to 3.8 billion years old, are found across the globe. Examples dating from around 2.5 billion to 2.1 billion years ago provide some of the oldest evidence of what’s known as the Great Oxygenation Event in layers of fine sediment and iron oxides.
Oxygen is highly reactive and will readily bond with other elements such as iron to produce iron oxide – the same substance as rust. Banded iron formations record when Earth rusted – when oxygen newly free in the atmosphere bonded to iron and other elements.
There was a lot of iron, so there was a long period where oxygen was being produced by cyanobacteria and then immediately reacting with iron to form iron oxides and being locked away. Only once all the iron was ‘used up’ could oxygen begin to build up in the atmosphere.
A beautiful banded iron formation on display in our Hintze Hall.
The impact of photosynthesis can’t be overstated. Our Principal Researcher Dr Paul Kenrick explains that, “photosynthesis was the first time anything tapped into an unlimited source of energy – the Sun”.
The oxygen released by photosynthesis allowed the evolution of aerobic respiration, which made life more productive.
“Everything scales up once you’re able to use energy from the Sun and you’ve got oxygen. This changed the face of Earth, enabling the evolution of complex life forms and the possibility of having much more complex ecosystems,” Paul adds.
But it wasn’t all positive. Oxygen is so reactive that it can damage some of the important enzymes within living organisms – even those that help photosynthesis happen. Plants had to evolve a way around this. For some organisms it was even more catastrophic. Oxygen was so toxic they were forced to retreat to environments that lacked it, such as deep-ocean sediments.
To go from simple, single-celled organisms to the abundance of complex species that exist today, life first had to reach key milestones.
For plants, the key step was when chloroplasts developed, around two billion years ago. Archaeal cells often consumed bacteria and, in some cases, the engulfed organisms survived and formed a symbiotic relationship with their host. That’s how we got mitochondria – which are cells’ energy centres.
When a cyanobacterium survived being eaten, it became the chloroplast, the engine of photosynthesis in plants. The green and red algae this created are the first organisms we might call plants. Surprisingly, this probably happened first in freshwater lakes or rivers, then later spread to the oceans.
It’s also from here that plants spread onto land, around 500 million years ago. Once there, they really thrived. Even today, plants make up more than 80% of Earth’s biomass – the living matter on our planet.
Paul explains, “plants inherit their biochemistry and cell biology from green algae. But all the visible things that everybody associates with plants today, such as stems, roots, leaves and flowers, evolve during or after the transition to the land environment. Plants are organisms of the atmosphere.”
Microfossils hint at when life went from single celled to multicellular. But animals don’t show up in any recognisable form until the Ediacaran Period, 635 million to 539 million years ago.
Recognisable may be a bit of a stretch – few creatures looked anything like those we see around us today. For example, there are beautiful fossils called rangeomorphs that look like fern fronds and, at first glance, appear more like plants than animals.
Rangeomorphs were enigmatic animals formed of self-similar branches. Image © Smith609 at English Wikipedia via Wikimedia Commonsopens in a new window, licensed under CC BY 2.5opens in a new window
But there are some clues. When looking for animals in the fossil record, our researcher Dr Imran Rahman says, “one thing that’s quite useful is finding a gut – that’s a good indicator that what you’ve got is an animal. Additionally, most animals alive today have bilateral symmetry, where you can divide their body into left-right mirror image halves, and we can find Ediacaran fossils that seem to show that same feature.”
Sponges – stationary animals that lack these classic animal features – also first appeared around this time. We can trace links between the earliest fossil sponges and modern species. In fact, Imran says, the more he and other researchers have looked at the ‘weird’ animals of the Ediacaran, the less odd they actually seem.
“The scientific narrative that surrounded Ediacaran fossils for a long time was that they were these weird, unknowable organisms that we couldn’t possibly make sense of. But in the last 5–10 years, we’ve moved beyond that. Now we can start to draw comparisons between these fossils and the animals we see in the rest of the geological record, and, indeed, today.”
After the Ediacaran, animals became much easier to recognise and interpret. The Cambrian explosion, starting from about 539 million years ago, was a period of rapid evolution. While some key animal groups may have appeared earlier, the Cambrian is when a number of major animal groups alive today first appeared.
A reconstruction of Ediacaran marine life, including numerous stationary leaf-like filter feeders. Image by Ryan Schwark via Wikimedia Commonsopens in a new window, licensed under CC0 1.0opens in a new window
Unlike plants, animals were already morphologically and ecologically complex in water, so they had a lot of work to do to transition onto land. Water and air are different in fundamental ways. When living out of water, an animal has to breathe air, cope with feeling heavier, move around without swimming, see in a medium that transmits light differently and so on.
Dr Richie Howard, our Curator of Fossil Arthropods, says, “to leave one world behind and embrace another, animals had to undergo enormous anatomical and physiological changes through millions of years of evolution to successfully make that transition.”
But it was worth it to exploit a whole new ecosystem.
“Getting in early is a recipe for success,” Richie explains. “You face less competition for resources and fewer threats from predators. Animal groups that successfully adapt to a new lifestyle or a new environment in this way often become abundant and diverse.”
That new ecosystem was created by plants and the first animals to make use of it were almost certainly arthropods – the group that includes insects, crustaceans, arachnids, (including spiders and scorpions) and myriapods (including millipedes and centipedes). The first land animal fossils we have are of myriapods and arachnids – from the Silurian Period, 443 million to 419 million years ago.
Asteroxylon mackiei was a land-based plant that lived during the Early Devonian Period, which lasted from roughly 420 million to 393 million years ago. The original fossil of this species was found in Rhynie Chert rocks in Scotland.
In the half a billion years following the Cambrian explosion, plants, animals and fungi spread to new niches and diversified into potentially billions of species. Here are some of the most remarkable milestones in the story of complex life on Earth.
Plants evolve a vascular system – a crucial innovation! It’s a way of transporting water and nutrients from the soil up through the stem, helping them to avoid drying out. These early land plants also have fungi to help them get important nutrients from the soil.
Roots and leaves develop, eventually resulting in trees. At this time, the world’s oldest forest is growing in what’s now Devon – in fact, it’s this county that gives the Devonian Period its name.
The first vertebrate animals follow arthropods onto land, with the charge possibly led by fish like Tiktaalik. This lobe-finned species had characteristics of tetrapods – four-limbed animals, which includes us. Tiktaalik likely used its sturdy fins to pull itself onto shallow shores.
In the Carboniferous Period, the first reptiles evolve from amphibians. Their innovation is laying eggs that can survive outside of water. Reptiles can also breathe air fully, meaning they can become fully land-living animals.
In the Triassic Period, mammals – or animals somewhere between reptiles and mammals – first appear. They’re evolving during the age of dinosaurs, when reptiles ruled the land, so most early mammals are rather small.
Dinosaurs are still dominating the world in the Jurassic Period. Many, such as Stegosaurus and Allosaurus, are getting big and bulky.
Some dinosaurs also start developing feathers, with animals such as Anchiornis and Aurornis evolving. Perhaps the most famous feathered dinosaur of all, the bird-like Archaeopteryx, appeared in the Late Jurassic.
Archaeopteryx lived 149 million to 145 million years ago in the Late Jurassic Period. It’s sometimes called the ‘first bird’ but scientists now think there are earlier bird ancestors.
It’s been hundreds of millions of years since plants first appeared, but now they’re developing flowers. Before this, most pollination was by wind, but, with the evolution of flowers, plants are now enticing insects, which have become abundant, to do the job instead.
Flowering plants may have their origins as far back as the Triassic, but it’s not until the Cretaceous Period that they really take off.
Finally, humans appear on the scene.
Our ancient ancestors split from apes around six million years ago, however, our genus Homo first emerged two million years ago.
We modern humans – the species Homo sapiens – are the last survivors in a long line of ancient hominins. Our species evolved in Africa, but exactly when is hard to work out. The oldest Homo sapiens fossils are around 300,000 years old, but our species could be much older than that.
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