When was evolution established as a legitimate science?

Evolution by natural selection is the theory that explains why animals change over time. This change has been witnessed in various examples, e.g gypsy moths, etc, and verified with DNA studies. "Legitimate" science is simply someone's opinion. Science is always someone's opinion. It is trying to look at the evidence and determine what is most likely true. Often theories are wrong and then other theories need to be made that better explain the evidence.

What is a "legitimate" science?

Earliest known scientific discussions about evolution of species can be traced back to ancient Greeks circa 600 B.C.

Unfortunately, as you know, ancient knowledge had been lost under European Christian Dark Age and only rediscovered thanks to Enlightenment.

A few milestones since then:

1751 Maupertuis concepts of heredity and "struggle for life" (aka "natural selection")

1809 Lamarck laws of use and disuse predicts which organs tend to develop or weaken over generations

1859 Darwin publishes On the Origin of Species, often considered a key point of biological evolution.

20th century genetics science confirms DNA is behind heredity.

That would be because evolution isn't a science.

Biology is a science and modern biology is based upon the theory of evolution

Evolution is not a discovery. It is an explanation for the appearance of related species over time and distance. Also you don't seem to know what legitimate science is. Legitimate science is not "that which has been verified".

You're using the wrong search parameters !

Also given the theory of evolution dates back to the 19th century the oldest science journals discussing it are probably in special archives to protect the now fragile paper the sort of archives you need authorization and proof you have a genuine research project to access

DavidJohnston29 is wrong and right. Gravity existed before Sir Isaac Newton posited his theories defining it. Evolution's 'discovery' (discovered in the same way as evolution) was, of course, Charles Darwin. I imagine the Wiki article on him will begin the trail of breadcrumbs as to who further developed his theory.

The scientific method requires that an experiment be repeatable so I don't understand what is scientific about evolution. It's just a bogus claim.

On the Origin of Species, published on 24 November 1859, is a work of scientific literature by Charles Darwin which is considered to be the foundation of evolutionary biology.

Evolution is a tool of science (specifically biology), not a separate scientific discipline.

Sciences tend to arise, with no significant point at which they can be called discovered. For instance, medical science began a very long time ago but had no inventor.