Life & Supercontinents

Humans

In the brief period of time since their origin, humans have had a large impact on Earth’s ecosystem.

Human activities are driving the current rapid loss of biodiversity – through climate change, habitat destruction, pollution, overhunting, and the spread of pathogens and invasive species.

Extinction rates over the last 500 years are higher than those of previous mass extinctions. If rates continue unchanged, in as little as three more centuries the amount of species lost can match those of previous mass extinctions – which took place over millions of years.

K–Pg Extinction

The asteroid impact at Yucatán Peninsula 66 million years ago marked the end of the Cretaceous period. The impact created gas and dust that blocked out the sun, caused acid rain, and acidified the oceans.

As a result, 75% of plant and animal species were driven to extinction, including all non-avian dinosaurs.

This was also the beginning of the Age of Mammals, who were able to diversify in newly available niches previously filled by dinosaurs.

Flowers

Flowers use their scent, shape, and colours to attract pollinators who in return spread their pollen. The rapid diversification of flowering plants coincided with the appearance of many new insect groups, as each side coevolved increasingly specialised features adapted for each other.

Many also develop fruits, which provide nutrients for seeds and encourage animals to eat and disperse them to new areas.

These advantages allowed flowering plants to quickly replace conifers as the dominant form of vegetation on land.

Dinosaurs and Mammals

The earliest dinosaurs appeared around the mid-Triassic period, though they would not become a dominant life form until after the mass extinction event at the end of the Triassic.

Dinosaurs breath with air sacs throughout their body, surrounding their organs and filling their bones. This allows them to grow to colossal sizes without overheating. This lightweight body structure also enables some species to fly, and is still seen today in birds – the only surviving dinosaurs.

Early mammals were small, nocturnal creatures who evolved in the shadow of the dinosaurs. They have a faster metabolism made possible by several adaptations: differentiated teeth that made digestion more efficient, a nose to enable breathing while chewing, and diaphragms that allowed more powerful breaths.

Combined with the development of ear drums that help them sense their surroundings in the darkness, mammals were able to find a niche as hunters in the night, when other animals with slower metabolisms move slower due to the cold.

Great Dying

The Permian-Triassic extinction event – commonly known as the Great Dying – was the greatest mass extinction in Earth’s history, killing 96% of all species.

The main cause was the large volcanic eruptions of the Siberian Trap, which released toxic gases, caused global warming, acidified the oceans, and reduced oxygen levels.

Amniotes

Unlike fish eggs, the amniote egg provides the embryo with a self-contained internal aquatic environment, surrounded by layers of protective membranes.

This adaptation allowed amniotes to spread inland away from coastal areas, unlike their amphibian ancestors who still needed to return to water to lay eggs. Amniotes alive today are commonly classified as either reptiles, birds, or mammals.

Seeds

The seed’s tough rind allows it to lie dormant for years, waiting to germinate in favourable conditions. The nutrient reserves inside also provide seedlings a better chance at surviving early development.

And with adaptations like wings and barbs, some seeds can be carried by wind or animals over longer distances compared to single-celled spores.

Seed plants also developed pollen, whose protective coat helps them spread their genes farther and reproduce in the absence of water.

These adaptations enabled seed plants to colonise inland areas.

Roots, Leaves, and Stems

The evolution of vascular tissue allowed plants to distribute resources throughout their body, enabling them to grow larger and develop even more specialised organs that help them survive on land.

Roots allow plants to reach water and nutrients stored underground. Plant roots changed Earth’s landscape by creating soil; before, dry land was a desert of bare rock and dust, prone to violent flash floods and temperature fluctuations.

Leaves specialise in photosynthesis, as their large surface areas enable more efficient capture of light and CO₂.

Stems allow plants to grow taller, giving them a competitive advantage in both sunlight access and spore distribution.

Land Plants and Animals

The first plants on land evolved from green algae that developed desiccation-resistant spores. These small, creeping, and moss-like plants live in moist environments near coasts, as they still depend on water to reproduce.

The first animals on land were small, air-breathing arthropods like centipedes or spiders, feeding on microbial mats in coastal environments.

Animals Diversify

Animals diversified in the oceans at an unmatched rate during the period commonly known as the Cambrian explosion, which saw the emergence of essentially every phylum of animal known today.

This was accompanied by the widespread development of hard body parts – like shells and skeletons – independently in different animal lineages, likely driven by increased predation pressure and availability of calcium in the ocean.

Complex Multicellular Life

The oldest macroscopic fossils of complex multicellular life come from the Ediacaran period.

Most of these soft-bodied organisms had very different body structures than life today, and it is hard to tell where they fit in the taxonomy of life.

“Boring Billion”

The period between 1.8 to 0.8 billion years ago is sometimes referred to as the Boring Billion, due to the stability in ocean and atmospheric composition as well as apparent slow diversification of life.

Although eukaryotes had emerged around the beginning of this period, the oceans remained dominated by single-celled prokaryotes. The development of multicellular life may have been limited by the scarcity of oxygen in the oceans.

Eukaryotes

All complex multicellular organisms like plants, animals, and fungi belong to the domain of Eukarya.

Unlike prokaryotes, eukaryotic cells contain a nucleus and organelles that perform specialised functions. This compartimentalisation enables more efficient energy use and allows them to attain larger sizes.

The most widely-accepted theory for their origin is symbiogenesis, whereby a bacterium becomes an organelle by genetically integrating with a host cell and sharing their functions. The mitochondrion and chloroplast are organelles that each descended from a single lineage of such endosymbiotic events.

Great Oxidation Event

Starting around 2.4 billion years ago, oxygen first began to accumulate in the atmosphere as the rate of oxygen production outpaced its sequestration.

Although oxygen-producing cyanobacteria had developed by 2.8 billion years ago, for a long time any free oxygen was captured by reactive compounds from Earth’s interior.

This influx of oxygen was significant enough to cause mass extinctions of anaerobic life. However, atmospheric oxygen continued to remain below 5% of modern levels until another major increase around 600 million years ago, coinciding with the development of Ediacaran life.

The increase in oxygen availability was a critical step towards the development of complex multicellular life, whose higher energy requirements depend on the additional free energy from oxygen.

Oxygenic Photosynthesis

Cyanobacteria were the first producers of oxygen. Although other early life captured energy from the sun via other types of photosynthesis, these were the first to produce oxygen as a by-product.

This innovation apparently occurred only once. In fact, chloroplasts are descendants of a single cyanobacterium that became genetically integrated as an organelle of its host cell. Today’s plants and algae all depend on the chloroplast to perform photosynthesis.

Oldest Fossils

The oldest fossils found are the stromatolites of the Strelley Pool Formation in Australia. These microscopic sedimentary formations have complex structures that cannot be explained by known non-biotic processes. They were likely formed by photosynthetic microbial mats that lived on the shallow ocean floors.

Water

Signs of crust and liquid water existed by 4.3 billion years ago, when Earth first became theoretically habitable.

Formation of Earth

Earth formed around 4.6 billion years ago along with other bodies of the Solar System. Shortly thereafter, around 4.5 billion years ago, a Mars-sized object collided with Earth, ejecting molten debris that became the Moon.