Could aliens have visited Earth before humans existed?

Yes, it is theoretically possible, but there is no accepted scientific evidence that aliens ever visited Earth before humans existed.

Earth is about 4.5 billion years old, while anatomically modern humans have existed for only about 300,000 years. So there was a very long window in which such a visit would have had to occur.

That said, Earth’s surface is constantly recycled by plate tectonics, erosion, weathering, and glaciation, so any very old physical trace would be hard to preserve. Scientists have also explored this idea in work related to the “Silurian hypothesis,” which asks whether we could detect an ancient nonhuman civilization after millions of years.

As of now, geologists and paleontologists have found no chemical signatures, artificial isotopes, geological anomalies, or other evidence to suggest an advanced alien presence ever visited pre-human Earth.