Origins of Life
🐸 Time-line of Life:
Signs of earliest life can be traced back to around 3.8 billion years ago, less than a billion after the Earth was formed. Bacteria and other prokaryotes were the first inhabitants of our planet. Complex Multi-cellular life only appeared about half a billion years ago, and it was bolstered by an event called the Cambrian explosion that took place from around 540 million years ago to 480 million years ago. Cambrian explosions saw the evolution of all sorts of life and most of the living beings today can be traced back to this time period.
Dinosaurs and Mammals (our ancient ancestors) only appeared 250 million years ago. It is believed that the Chicxulub asteroid was responsible for the death of the dinosaurs, where the smaller mammals survived. The extinction of dinosaurs was extremely important for the emergence of human civilization. Homo sapiens (us humans) have only been around for 500,000 years. This timeline puts into perspective that we have just popped up in the history of our own planet much less in the history of the universe.
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🦠 Panspermia:
We still do not know how life arose. It could have originated here on Earth or it may have traveled from outer space to our planet. Panspermia represents the idea that life did not originate on our planet (Arny and Schneider). It originated somewhere else and then accidentally traveled to Earth. For example, life could have originated on mars. Unlike complex multicellular organisms, simple life (like bacteria) can survive in space for a long time. Mars and Earth may have exchanged debris due to collisions in their young days. These collisions resulted in life transferring to Earth. This is a good thought experiment, but it doesn't answer the question. It just transfers it from one place to another. For example, one can ask how did life originate on Mars? and so on.
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🐳 How did life start on Earth?
In 1953, graduate student Stanley Miller and his professor Harold Urey experimented to inspect the origins of life on our own planet. They filled a flask with water, ammonia, methane, and hydrogen. These gases are thought to be abundant in the early days of our planet. After running an electrical current through the mixture, they discovered organic molecules and some of the amino acids used by living organisms to make proteins (Arny and Schneider). The ingredients required for life could have been made in the atmosphere of the early earth by something as mundane as lightning strikes. From the Miller-Urey experiment, it would make sense to conclude that where ever the ingredients for life are present, life will follow quickly.
We should think about this conclusion with caution because we only have one reference point; Life on Earth. We have no other data for comparison. It may be a statistically unlikely event. For example, let's say you buy your lottery ticket and win the prize. You cannot assume that you will win this prize every time you buy the ticket, because what just happened is an extremely rare event. In the same way, we cannot assume that life will form where ever the ingredients for carbon-based life are present.
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[1] Arny and Schneider, Explorations: An Introduction to Astronomy.
[2] Arny and Schneider, Explorations: An Introduction to Astronomy.
