Above: Image © kowalska-art, iStockphoto.com

In September 2017, scientists published an exciting new finding: fossils dating back to 3.95 billion years ago. And just a few months earlier, scientists had reported on ancient fossils from the Nuvvuagittuq Greenstone Belt near Inukjuak, Quebec. These fossils date back from 3.8 to 4.3 billion years ago!

These are the oldest fossils that people have ever found. Were these the earliest things ever to have lived? Scientists don’t know. But it’s still interesting to think about what these early life forms meant.

After all, they were the ancestors of every living thing around you: the squirrel outside your window, the flowers and worms in your garden, the bacteria and fungi in the soil in your backyard...even you!

Did you know? By comparing the DNA sequences of two organisms, scientists can estimate how much time has passed since they last shared a common ancestor.

Billions of years have passed since the earliest living beings were around. Since then, your ancestors have adapted to global climate change, found creative solutions to ecological problems, and survived mass extinctions.

Bacteria & Archaea

The earliest living things were prokaryotes (single-celled organisms that lack a membrane-bound nucleus). They fall into two major groups: bacteria and archaea. Scientists don’t know which group came first because it’s hard to tell their fossilized remains apart.

The Oxygen Crisis

The first organisms found themselves in a challenging environment. Volcanic eruptions rocked the Earth’s surface. The hot, choking atmosphere was full of toxic gases like carbon dioxide, ammonia, and methane. Through chemical reactions with the carbon, sulfur, water, and metals around them, these creatures supported themselves with anaerobic (oxygen-free) metabolism.

But eventually, small organisms called oceanic cyanobacteria developed photosynthesis. That’sthe ability to convert sunlight into chemical energy.

Once cyanobacteria started photosynthesizing, they produced huge amounts of oxygen as waste. Oxygen was toxic to many of the anaerobic organisms. Its in the atmosphere caused many of these organisms to die.

Did you know? Anaerobic organisms still exist, but most live in oxygen-free environments. For example, they might live far underground or in the deep ocean.

Oxygen also changed Earth’s climate. Previously, the Earth’s atmosphere contained the strong greenhouse gas methane, which insulated the planet. Oxygen converted that methane into the weaker greenhouse gas carbon dioxide. With less insulation, heat escaped into outer space more easily. Eventually, this caused a global cooling event and an ice age.

Perhaps most importantly, the surge in oxygen levels paved the way for a new energy-producing pathway: aerobic metabolism.

Aerobic Metabolism & Multicellular Life

Aerobic metabolism uses oxygen to produce energy. It’s far more powerful than anaerobic metabolism. Organisms that use aerobic metabolism can extract up to 15 times more energy from a piece of food. In other words, an anaerobic organism could get 20 calories from a meal. Meanwhile, an aerobic organism could get 300 calories from that same meal!

Small prokaryotic cells that could use oxygen were engulfed by other cells and put to work. These engulfed cells eventually evolved into the mitochondria and chloroplasts that power animal and plant cells today.

Some scientists think that this made cells more efficient. Because of this, they could grow larger and more complex over time. For example, they evolved nuclei to act as control centres. They also evolved endoplasmic reticula to act as protein and fat factories.

Sometime between 1 billion and 800 million years ago, individual cells began to band together, specialize, and work as a unit. That’s when multicellular organisms appeared.

Dunkleosteus Skull
Dunkleosteus Skull

The Invasion of Land

Fast forward to 380 million years ago, the late Devonian era. Multicellular life has evolved by leaps and bounds. The ocean is dominated by invertebrates, reefs, and trilobites. Early sharks and fishes are abundant. Some look like sea monsters! Dunkleosteus was the size of a school bus and covered in bony plates, and snapped like an alligator.

In this “age of fishes,” the ocean was oxygen-poor and highly competitive. Before this, waters housed tetrapods. These were four-limbed vertebrates that would eventually become mammals, birds, reptiles and amphibians. But the lack of oxygen drove some tetrapods out of the water. Skeletons and muscles became stronger to support body weight in air. Gills turned into lungs. Fleshy, lobe-like fins became bony limbs. A 2017 study suggests that when fish developed larger eyes, they could visualize opportunities on land.

The Age of Dinosaurs & Rise of Mammals

The invaders did well on land, and the diversity of these organisms exploded. About 310 million years ago, they split into two groups: the sauropsids and the synapsids. The sauropsids were the ancestors of dinosaurs and reptiles. The synapsids were the ancestors of modern mammals.

The Great Dying, the most severe mass extinction event in the last 600 million years, nearly wiped out both groups. This happened roughly 250 million years ago. No one knows exactly what happened. Scientists have suggested some theories. Some scientists think there was a major asteroid impact. Others blame volcanic activity. Others suspect drastic changes in ocean chemistry.

In the void left by extinction, the remaining sauropsids were wildly successful. The earliest dinosaurs were small, walked on two legs, and had grasping hands.

Dinosaurs diversified over time. Some groups, like the long-necked sauropods and horned triceratops, adapted to walking on four limbs. Many had feathers, and some may have had limited control over their body temperature.

Dinosaurs generally became larger as they evolved. They reached their peak size in the late Jurassic and Cretaceous periods. Some were over 35 metres long and weighed 100 tons!

Meanwhile, synapsids from this time were small and nocturnal. Scientists believe that they developed warm-bloodedness 215 to 199 million years ago, in the late Triassic period. This allowed them to maintain a stable body temperature in all environments.

This age ended disastrously 65 million years ago. The Cretaceous-Tertiary extinction wiped out most dinosaurs and other giant reptiles. Scientists think this extinction was caused by an asteroid impact.

Suddenly, there were abundant opportunities for other creatures to explore and evolve. The way was clear for other lineages, especially mammals, to dominate the planet.

Looking back, it’s amazing how far life has come from the humble origins of the last common ancestor.

Did you know? So far, there have been five mass extinction events. Some scientists argue that human activities on Earth have begun a sixth mass extinction event called the Holocene or Anthropocene extinction.

Learn more!

Brittney G. Borowiec

I completed my undergraduate degree in Honours Biology (Physiology Specialization) at McMaster University, located a short trip down the 403 from my hometown of Mississauga, ON. I’m now a PhD candidate in the same department, studying how fish respond to different patterns of low oxygen stress. When I’m not volunteering for Let’s Talk Science, you can find me blogging about oceanography at Oceanbites.