The most widely held theory about how humans came to populate Earth is the Out of Africa theory.

Here's why:

* Strongest Evidence: This theory is supported by extensive genetic and fossil evidence.

* Genetic Similarity: Studies on human DNA have shown that all humans share a common ancestor who lived in Africa around 200,000 years ago.

* Fossil Record: The oldest known human fossils are found in Africa, and early human migrations out of Africa are documented in fossil records.

While other theories exist, such as the multiregional hypothesis, the Out of Africa theory is considered the most scientifically credible and widely accepted explanation for the origin and spread of humans.