First form of life
According to Darwin's theory of evolution all life forms originated from one common ancestor. This chapter will look at the first life forms on Earth. It will discuss the environmental factors that led to the formation of the first organic molecules from inorganic substances in the process of abiogenesis. Millions of years later these organic substances evolved into the first living organisms.
After decades of carefully studying the meteorites' fragments, rocks and fossils, scientists were able to establish the approximate age of our planet. Today it is believed that Earth was formed about 4600 million years ago. For thousands of years it was just a ball of hot, liquid rock floating in space. Eventually Earth's surface started to cool and harden. Earthquakes, volcanic eruptions, comets and meteors were the main forces shaping our planet at that time. Water and air were still non-existent.
So, how did this boiling mass of rocks, lava and gas became a planet with oceans, rivers and forests inhabited by millions of different life forms?
The transformation took millions of years. To become a life-sustaining place, Earth had to cool down and acquire some water. One of the water-origin theories today states that first Earth's first rain was just a cooled-down volcanic steam. This rainwater started to collect in low-lying areas of the Earth's crust, forming the first ocean.
Another theory states that the water in our oceans might have been delivered by massive ice-bearing comets. See image 1.
Formation of the ocean was an important step towards creating the first life forms but it was not enough.
Primordial Earth had very little free oxygen and no ozone layer to absorb ultra-violet (UV) radiation from the sun. These conditions were not suitable for living but they were great for speeding up chemical reactions that otherwise would not have happened. It was at that time that the first organic molecules were formed from non- organic elements. The formation of living forms from non-organic matter is called abiogenesis.
Laboratory experiments by scientists like Oparin, Miller and Urey have confirmed this theory of abiogenesis. These scientists have re-created (the early) Earth's environment in a laboratory. When gases containing inorganic elements, like hydrogen, oxygen, nitrogen and carbon were heated with water and energised by electrical discharge or by UV radiation, they formed small organic molecules. See image 2.
First biological cells
These first, simple organic molecules accumulated in the ocean for millions of years, producing a so-called 'organic soup'. Fuelled by the still-extreme heat of early Earth, these molecules were interacting with each other, producing new, more complex organic compounds.
Some groups of organic molecules formed bubble-like structures called coacervates. Coacervates were made of organic molecules surrounded by a film of water molecules. Coacervates could selectively absorb different materials from the surrounding water and incorporate them into their structure. Some of these coacervates started to group, grow and divide. Millions of years later some of them evolved into true biological cells. A true cell can be characterised by the following:
- it replicates itself, generation after generation;
- it has a set of complex proteins, essential for biochemical reactions;
- it has a membrane that separates the cell from its surrounding environment and enables it to maintain a distinct chemical identity.
See animation 1.
The oldest fossils
Stromatolites are the oldest known fossils. They are about three billion years old. Stromatolites are column-like structures formed by the trapping, binding or precipitating of sediments by different types of ancient microorganisms. They are made from layers of so-called microbial mats that are layered groups, or communities, of microbial populations. See image 3.
The first organisms were unicellular prokaryotes or, in other words, their whole body consisted of one cell that did not have a well-defined nucleus. It is likely that early cells were heterotrophs. These heterotrophic cells would have obtained their energy from surrounding organic molecules in the process of fermentation. Because of the early Earth's conditions they would also have been anaerobic, meaning they did not need oxygen to survive. These organisms would also have thrived in high temperatures, meaning they were hyperthermophilic. These first prokaryotic 'extremists' (because of their extreme living conditions) are classified as Archaeabacteria by some scientists. According to another group of scientists, they fall into a group separate from bacteria, called Archaea.