The Final Boss of Earthian Lore:
Long ago, something hit the Earth. This collision spawned the moon (moon is said to have the same composition as the Earth) and transferred large amounts of energy to the Earth, possibly creating the tectonic plates or at least keeping the magma underneath the tectonic plates very hot and active.
As a result of the enhanced magma activity and dynamism of our tectonic plates, the Earth has a very wide range of diversity of biomes: deserts, glaciers, forests, plains, etc., all come in dozens if not hundreds of variants.
The diversity of biomes created a massive diversity of bacteria, plants, insects, etc. Basically, a very rich biosphere. Uniquely rich, actually, since it took a massive collision to create it.
Humans evolved, or possibly were engineered, in this very rich biosphere and have adapted to it. Humans also come in thousands of variants, due to the wide exposure to different biospheres.
Overall, this is a biological treasure. Other planets hosting life, wouldn't have such a huge diversity in their biology - Earth may be unique in this corner of the galaxy.
Some say that foreign species come here to study and use humans to harvest antibodies for their medicine.
To me, this could make sense. We actually do the same with insects, plants, and animals - we study them all and find weird genetic characteristics (e.g., lizards or fish that can be frozen and revived) and use that to develop our own medicine.
I've never heard this idea in American alien lore. It came from Jean-Pierre Petit and was only in French.