It's tricky to pinpoint the *exact* species that appeared first on Earth, as the very early stages of life are incredibly difficult to study and leave limited fossil evidence. However, we can say that the earliest life forms were likely single-celled organisms called prokaryotes. These included:

* Bacteria: These are incredibly diverse and adaptable, found in almost every environment on Earth.

* Archaea: These are similar to bacteria in appearance, but have unique biochemistry and often live in extreme environments.

Scientists believe these early prokaryotes emerged around 3.5 to 4 billion years ago, not long after the Earth's formation.

It's important to note:

* The exact species and their evolutionary relationships are still being studied and debated.

* The earliest life forms were likely very simple and evolved over time into more complex forms.

It's amazing to think that these simple organisms were the ancestors of all life on Earth, including ourselves!