How did microbes appear on earth?

Like many questions in science, the correct answer is "we don't know".

What we do know for sure is that the Earth is about four and a half billion years old. The earliest fossils of lifeforms we have are microbes from about three and a half billion years ago. Therefore microbes were present on Earth when the Earth was only a billion years old. There is no evidence to show how these microbes arose. Anything you read on the subject is speculation.

Another interesting fact is that the Earth was populated entirely by microbes for three billion years. It is only in the last half a billion years, called the Cambrian period, that complex plants and animals developed.

So small you cant see them at ALL----until we got Microscopes.

The first living things were not organisms at all.  They were self-catalyzing, self-replicating molecules, perhaps a ribonucleic acid.   These RNA molecules evolved over millions of years into the first cellular organisms.

Microbes first appear on earth about 4 billion years ago.

when primitive bacteria appeared in sulfurous oceans under poisonous skies

Microbes appear just as they always have. 

All dressed up, and nowhere to go.

What does this have to do with astronomy and space?

It is called Evolution.    Life on Earth Started in the Oceans.

There are several competing scientific theories of biogenesis.  There are reasons for and against all of them.

So how did the meteor microbes come about? Whatever answer you come up for meteors, would work even better for microbes first appearing on Earth. Unless you can somehow show that super-cold, dry space is better than a warm, wet Earth.

Another explanation is that they formed in the chemical soup (somtimes called the primordial soup) of the early CO2-rich and O2 depleted earth.  Literally, chemistry run amok.