Ancient Greeks thought that life on Earth originated from non-living things, and this was happening all the time. For example, they held the opinion that mice came from straw; or rotting meat transformed directly into flies, and flies themselves were created from sand. Spontaneous generation is the term that was used to describe the creation of life in this way. The Greek scientist and philosopher Aristotle (384-322 BCE) was one of the early scientists to proposed that life arose directly from different combinations of earth, wind, fire, and water.
Today, although we still do not conclusively know how life has arisen on Earth, we have several hypotheses.
When Has Life Emerged?
Life is old. The non-avian dinosaurs are perhaps the most famous extinct creatures, and they had their beginnings 250 million years ago. But life dates back much further.
Here is the estimated timeline of life’s evolution on Earth:
- modern humans emerged 0.2-0.3 million years ago.
- humans emerged 2 million years ago.
- apes emerged 50 million years ago.
- mammals emerged 210 million years ago,
- amphibians emerged 370 million years ago,
- multicellular emerged organisms 600 years million years ago,
- life itself emerged 3700 million years ago
Here are two of the prevailing hypothesis on the origin of life; the first one proposes that life emerged in deep oceans, and the latter proposes that life was planted on the earth by a colliding meteor or comet 3.7 billion years ago.
Deep Hot Biosphere Hypothesis
If we assume that life formed on Earth, which seems reasonable, given that we have not yet found it anywhere else then it must have done so in the billion years between Earth coming into being and the preservation of the oldest known fossils. Earth formed around 4.5 billion years ago, and the first known fossils are 3.7 years old, which means life began when the surface of Earth was hostile to life.
Deep Hot Biosphere Hypothesis was first proposed by astrophysicist Thomas Gold. The Hypothesis suggests that life first emerged a few kilometers below the surface of Earth under high pressure and temperature. These organisms are deep in the Earth in ‘micro-habitats’, and their food source is supplied thanks to out-gassing by the crust of the Earth.
The leading form of evidence for this hypothesis comes from the discovery of ‘nanobes’ (organisms that are smaller than bacteria but contain DNA) in deep rocks.
The main concept of Panspermia is that life, or the fundamental building blocks of life, exists throughout the universe. This hypothesis posits that life arrived on Earth by ‘piggy-backing’ on an asteroid, planetoid, or meteoroid. This ancestral form of life would have been delivered to Earth by the impact of the said meteoroid, planetoid, or asteroid with the surface of the Earth.
Panspermia Hypothesis bases itself on life existing on extrasolar bodies in a vacuum that travels through the universe. Therefore, this field of research requires a lot of investigation into extremophiles on Earth that can overcome challenges such as solar radiation exposure in a vacuum. One such organism is the tardigrade.
Tardigrades are an extremely interesting organism that has fossils dating back to 530 million years ago. They are important to the hypothesis of Panspermia because they have been shown to survive being exposed to absolute zero temperatures (273 C, which is the temperature in space). They can resist 1,000 times more radiation than any other animal known, and have been shown to survive without water for almost a decade. They are also the first animal shown to survive the vacuum of space
Is Evolution Theory About the Origin of Life?
While the evolution theory does encompass ideas and evidence regarding life’s origins, this is not the central focus of evolution theory.
The theory primarily deals with how life has changed after it has come into being. When one refers to the evolution theory as an established theory she means the evolution of life once it has materialized and not the materialization of life itself.
Darwin’s evolution theory is about the origin of species, which also happens to be the title he chose for his first book where he explained his theory in detail.