Life on Earth

Earth is the only planet in our Solar System that supports life. Our planet is home to an incredible diversity of living organisms. Millions of species are alive on Earth today – not counting those extinct or undiscovered!

How did a small, rocky planet become a unique hotspot for life? It's all to do with a lucky set of physical conditions.

Physical Conditions for Life on Earth

Life first evolved in water. Without liquid water, Earth would be a barren, lifeless planet. How do Earth's physical characteristics maintain liquid water on the surface, and other conditions required for life?

The Atmosphere

The mass of the Earth and the force of gravity retain an atmosphere above the surface of Earth. The atmosphere provides gaseous resources, such as:

• Carbon dioxide (required for photosynthesis)

• Oxygen (required for respiration)

• Methane

• Nitrogen

Additionally, atmospheric pressure helps maintain liquid water. Oceans cover over 70% of the Earth's surface, providing habitats for millions of species.

Fig. 1 – Clouds are made of liquid water droplets suspended in the atmosphere. They insulate Earth against temperature extremes and maintain the hydrological cycle. Unsplash

Insolation

Earth's temperature range is controlled by incoming insolation and the natural greenhouse effect. Gases like carbon dioxide and methane trap solar energy in the atmosphere, warming the planet. Without the natural greenhouse effect, Earth's surface would be about 30ºC colder, with most regions unable to support life.

Position in the Solar System

The position of Earth in the Solar System plays an important role in Earth's temperature. Earth exists in the 'Goldilocks Zone', maintaining a moderate distance from the Sun. It's not too hot or too cold. Earth's position enables temperatures that support liquid water, and therefore, life.

Moreover, Earth's orbital behaviour impacts insolation and temperature. Earth sits at a tilt of 23.5°, spins on its axis every 24 hours, and orbits the Sun every 365 days. Exposure to solar radiation varies, thus impacting temperature.

Magnetosphere

Earth has a molten core of iron and nickel. These metals produce a magnetic field, known as the magnetosphere. The Earth's magnetosphere deflects solar radiation and harmful ions.

The First Life on Earth

The oldest evidence of life on Earth comes from stromatolites.

These fossils are believed to be formed by aquatic photosynthetic prokaryotes, similar to modern-day cyanobacteria.

The oldest stromatolites found are 3.5 billion years old, found near the Marble Bar in the Pilbara region of Australia.

Where Did Stromatolites Come From?

It's not clear how life first came about on Earth. There are three main theories: primordial soup, deep-sea hydrothermal vents, and meteorites.

The Primordial Soup Theory

First voiced in a personal letter by Charles Darwin, the primordial soup theory states that if energy is added to the gases of Earth's early atmosphere, the building blocks of life would be created in the form of amino acids.

Potential energy sources for protein synthesis came from lightning strikes or UV radiation, common on early Earth.

The primordial soup theory was tested by Miller and Urey in 1953. They added electric sparks (mimicking lightning) to a mixture of methane, hydrogen, ammonia, and water. This experiment formed organic biomolecules.

However, there are some criticisms of the Miller-Urey experiment:

• It did not synthesise all amino acids found in living organisms

• It didn't show how life came about from the amino acids

• The experiment did not account for other gases present in the early atmosphere

The Deep-sea Hydrothermal Vent Theory

Alternatively, the hydrothermal vent theory states that organic compounds were first produced in deep-sea hydrothermal vents. These seafloor vents heat water and exude minerals from the Earth's interior.

Alkaline vents have a high pH (9-11) and release warm water (40-90ºC). When combined with the volcanic atmosphere, synthesis of organic molecules may have been possible.

The Meteorite Theory

Some scientists believe that organic molecules arrived on Earth from space.

Life on Earth Before Humans

When photosynthetic prokaryotes evolved, they produced oxygen as a by-product during photosynthesis. For several million years, the oxygen dissolved into the water. Around 2.7 billion years ago, the water became saturated, so the oxygen began to 'gas out' and enter the atmosphere.

Atmospheric oxygen increased gradually for a few million years, then shot up rapidly. This 'oxygen revolution' had an enormous impact on life. It likely caused the extinction of many prokaryotes, but enabled the fascinating and complex evolution of eukaryotes.

Eukaryotes first appeared 1.8 billion years ago, and the first multicellular eukaryotes (red algae) around 1.2 billion years ago.

Fig. 2 – Red algae off the coast of Norway. Unsplash

Soft-bodied animals first appeared approximately 600 million years ago, but it wasn't until the Cambrian Explosion of 543 million years ago when animals suddenly diversified. They developed hard bodies, new defensive adaptations, and compound eyes.

After the Cambrian Explosion, eukaryotes began to colonise land. Tetrapods have only been around for 365 million years – and Homo sapiens just 195,000 years.

Different Life Forms on Earth

All life forms are categorised into one of five kingdoms:

• Prokaryota (prokaryotes)

• Protoctista (protoctists)

• Fungi

• Plantae (plants)

• Animalia (animals)

Let's look at these kingdoms in more detail.

Fig. 3 – How many of the five kingdoms can you spot in this photograph? Unsplash

Prokaryotes vs Eukaryotes

All organisms are either prokaryotes or eukaryotes, depending on their cells.

Prokaryotic cells are small and simple. They lack membrane-bound organelles; their DNA is stored in small circular molecules called plasmids. Most prokaryotic organisms are unicellular.

Eukaryotic cells are larger and more complex. They contain membrane-bound organelles; their DNA is stored in chromosomes inside the nucleus. Eukaryotes can be unicellular or multicellular.

Life on Earth: Timeline

It's hard to summarise billions of years of evolution into a single table, so here are some key points.

I hope that this article has clarified how Earth's physical conditions affect life. Earth's unique position in the Solar System, its atmosphere, and its magnetosphere have enabled temperatures maintaining liquid water. Life first evolved in the water, gradually diversifying and colonising land.

^{1. NASA, What is the greenhouse effect?, 2022}

^{2. National Geographic Society, Axis, 2022}

^{3. Neil Campbell, Biology: A Global Approach Eleventh Edition, 2018}

^{4. Water Science School, How Much Water is There on Earth?, USGS, 2019}