"First emergence" refers to the initial appearance or occurrence of something, particularly when describing the first time something is noticed or seen. It implies a debut or the starting point of something new. Full definition
The gold deposits in the Witwatersrand, which are unique worldwide, could have thus been formed only during a certain period of the Earth's history: after the development of the first continental life forms in shallow lakes at least 3 billion years ago, but before the first emergence of free oxygen in the Earth's atmosphere approximately 2.5 billion years ago. (sciencedaily.com)
In the most widely accepted model of human evolution today, the first emergence of hominins out of Africa involved Homo erectus, and happened some time after 2m years ago. (scientificamerican.com)
This is the 50th anniversary of H3N2's first emergence, which occurred in Hong Kong in 1968, according to the CDC. (health.com)