How did cells first appear on earth?

The simple answer is, we do not know for sure. But there are several theories that pose a solution. I will list #2# of the more popular ones below:

1) Life evolved by accident, from molecules forming randomly in the ocean. This is the theory that most scientists today believe in today. It is believed by some scientists that the accidental creation of a single molecule of self-replicating RNA (similar to DNA) in the oceans of the world led to the eventual creation of cells.

2) Life came from outside of Earth. This is a theory believed by many science fiction fans and even scientists alike. It is possible that life never originated on Earth (maybe instead, it originated on Mars or an asteroid), and some of this life crashed upon Earth's surface in the form of a meteor. Of course, we are not talking about little green men, but rather unicellular bacteria. It has already been proven that some bacteria can survive in the vacuum of space when frozen, so this theory (however unlikely) might still believe how life began on Earth.

Those are the #2# more popular theories, but others exist. Of course, I didn't mention ideas like a superhuman being or beings (god(s)) being responsible for life on Earth, because there is not much scientific evidence supporting these ideas.

I hope that helps!

For the first cell to appear it had to have both proteins to protect it from the environment and a means of replication to reproduce the necessary proteins and enzymes.

The idea of a simple cells occurring easily by chance has long since be abandoned as the complexity and level of information found in the cell has been discovered.

One proposal is that life started from RNA. RNA first advocates propose that RNA performed both the enzymatic functions of modern proteins and the information storage function of modern DNA. Making RNA nucleotide bases has proven either extremely difficulty or impossible under realistic prebiotic conditions. Also the ability of RNA to function as proteins is extremely limited.

The DNA first model was proposed by Henry Quastler, in 1964. The problem is the incredible odds of getting information on the DNA and corresponding proteins at the same time and same place. (Yockey Information Theory and Molecular Biology p 247)

The coacervate protocell theory of Aleksandr Oparin dates from the 1920's proposed that proteins came first. The idea that soap bubble like circles of proteins formed the first cells. Then the proteins formed a system of self replication. As the complexity of DNA and proteins became apparent Oparin proposed that natural selection was able to work on prebiotic structures (Oparin Genesis and Evolutionary Development of Life p 146. ) Oparin's theory was attacked because proteins are not known to code for DNA or information.

None of the theories Protein first, DNA first or RNA first are able to explain the origin of complex information required for life. The first cells had to have both complex proteins and complex information needed to replicate both the proteins and the information.

The information required for life seems to require intelligence. There is no scientific mechanism for the idea of intelligence first either.