How Did Life Begin

In this section:

How did Life Begin on Earth
First Known Life on Earth
Common Theory of Complex Life Formation
Single Celled to MultiCellular Organisms

How did Life Begin on Earth:

The simple answer is we don't know. There are several theories on how life began and I have listed a brief summary of a few below. No Theory has been proven and probably never will because the chances of finding evidence of the earliest life on Earth is extremely unlikely.

Lightning - Electricity can produce amino acids and sugars from simple elements in the atmosphere. This theory says lightning may have created the building blocks of life. Then over a long period of time, larger and more complex molecules formed.
Deep-Sea Vents - Deap-sea vents erupt hot fluid that contain key elements to life. This theory says these elements came together to form the first life on earth.
RNA World - RNA is found in genes today and some scientists think this formed on it’s own first. The complex RNA world led to life forming.
Panspermia - Panspermia is the idea that life began elsewhere in the universe and traveled to Earth. The most common theory is that it hitched a ride on an asteroid or comet that hit Earth.

First Known Life on Earth:

The oldest known fossils that most scientists agree on are 3.5 billion years old. They are stromatolite-forming cyanobacteria, a species of blue-green algae, found in the Dresser Formation fossil site of the Pilbara region of western Australia. Early simple life forms are called Prokaryotes and below are some charateristics of these.

Prokaryotes are single celled life forms that lack a nucleus and are little more than tiny bags of chemicals.
For the first 2 billion years of life on Earth, it was simple (prokaryotes)
Prokaryotes never form anything more complex than chains or colonies of identical cells.

Common Theory of Complex Life Formation:

Scientists have a hard time coming up with how and why life became complex. The jump from simple life (prokaryotes) to complex life (eukaryotes) is a huge leap that currently cannot be explained.

A single celled eukaryote (example is an amoeba) is 15,000 times larger than a prokaryote. And Eukaryotic cells have miniature organs called organelles, internal membranes, skeletons and transport systems. They are way more complex the prokaryotic cells.

So how did simple prokaryotic cells evolve into complex Eukaryotic cells?

Originally scientists thought simple cells slowly evolved into more complex cells. But there are a couple of problems with this theory.

If simple cells had slowly evolved into more complex ones over billions of years, all kinds of intermediate cells would have existed and some still should. But there are none.
Prokaryotes cannot produce enough energy to grow complex enough to evolve into eukaryotes.

The main points of the commonly accepted theory are:

One prokaryote engulfed another and found some symbiotic benefit that kept the relationship going. The host cell is unknown but it engulfed a bacterium, which began to live and divide within it. It eventually formed a symbiotic relationship called endosymbiosis.
Through evolution over time, the endosymbionts eventually become an organelle called the mitochondrion.
Mitochondria allow the cell to produce its own energy which allows them to accumulate bigger and more complex genomes. These include the nucleus, which holds genetic information in the form of DNA and the endoplasmic reticulum, which shunts proteins and lipids around the cell. This eventually led to all the components of complex life.
The problem is there is no evidence for any of this, it is only theory. The only evidence is prokaryotes existed first for 2 billion years and then suddenly eukaryotes existed. No evidence of any evolution from one to the next.

Single Celled Organisms to MultiCellular Organisms:

Just like the leap from simple to complex life, the leap from unicellular to multicellular life seems almost impossible. Scientists still debate how unicellular life evolved to multicellular life.

One mystery about multicellular organisms is why cells did not return back to single-celled life. Unicellular organisms still exist and are very successful, in fact they are more abundant than multicellular organisms. So what is the advantage of being multicellular? If there is no advantage than multicellular life should not have continued to evolve. One theory of why multicellular life stayed is “ratcheting mechanisms”. Ratcheting mechanisms are traits that are beneficial in a group but are detrimental to lone cells.

Here is a list of some of the theories of how single cells evolved into multicellular organisms. These are brief overviews of each theory.

Symbiotic Theory - Different species of single celled organisms formed a symbiotic relationship with each other. Basically meaning they worked together. Over time they became so dependent on each other that they would not be able to survive independently. This eventually led to them becoming one multicellular organism. The problem with this theory is scientists don’t know how each organism’s DNA could be incorporated into one single genome to constitute them as a single species.
Cellularisation Theory - A single unicellular organism developed internal membrane partitions around each of its nuclei. The problem with this theory is there is not a good explanation of why the partitions would have formed.
The Colonial Theory - Many organisms of the same species formed a symbiotic relationship and over time became so dependent on each other that they were unable to survive independently. This eventually led to a multicellular organism. This is the more widely accepted theory. The problem with this theory is it is unknown how each organism's DNA could be incorporated into a single genome that would constitute a single species. This is the same as the symbiotic theory except that the same species formed a symbiotic relationship instead of different species.

One of the problems with Symbiotic and Colonial theories is they require single celled organisms to live in groups. Single cells living together in groups at the beginning of multicellularity puts them at a fitness disadvantage. Fitness is the ability of a cell to thrive in a given environment. So scientists struggle with why this happened.

All of these are just theories and there is no evidence for any of them. There are very few fossils from early life on earth to confirm or deny these theories. Scientists do laboratory tests and mathematical equations to try and test the theories.