It's impossible to definitively say what the first microorganism on Earth was, as we don't have fossils from that early period. However, scientists believe that the earliest life forms likely resembled single-celled prokaryotes, similar to modern-day bacteria and archaea.

Here's what we know about the early Earth and the emergence of life:

* Early Earth: The Earth formed about 4.5 billion years ago, but it was a very different place than it is today. The atmosphere was much thinner, and there was little to no oxygen.

* First Life: The first signs of life are thought to have emerged around 3.8 billion years ago. These early life forms would have been very simple, likely relying on the chemical energy available in the environment (like hydrothermal vents or volcanic activity).

* Microfossils: The oldest known fossils of microorganisms are about 3.5 billion years old. They are mainly found in sedimentary rocks and are often very small and difficult to identify.

* RNA World Hypothesis: Some scientists believe that early life may have been based on RNA, a molecule similar to DNA, instead of DNA. This is because RNA has a simpler structure and can act as both a carrier of genetic information and a catalytic enzyme.

Key Points:

* The earliest life forms were likely single-celled prokaryotes.

* We don't have direct evidence of the very first microorganisms.

* The emergence of life was a gradual process that took place over millions of years.

Research continues to uncover new information about the origins of life on Earth. It's an exciting area of study with many unanswered questions!