Science does not deal with "proof positive" but rather evidence. Nor does the term "missing link" have any real meaning. Science prefers "transitional."
This is a transitional, and it is evidence for the theory of evolution:
Site: Koobi Fora (Upper KBS tuff, area 104), Lake Turkana, Kenya (4, 1)
Discovered By: B. Ngeneo, 1975 (1)
Estimated Age of Fossil: 1.75 mya * determined by Stratigraphic, faunal, paleomagnetic & radiometric data (1, 4)
Species Name: Homo ergaster (1, 7, 8), Homo erectus (3, 4, 7), Homo erectus ergaster (25)
Gender: Female (species presumed to be sexually dimorphic) (1, 8)
Cranial Capacity: 850 cc (1, 3, 4)
Information: Tools found in same layer (8, 9). Found with KNM-ER 406 A. boisei (effectively eliminating single species hypothesis) (1)
Interpretation: Adult (based on cranial sutures, molar eruption and dental wear) (1)
See original source for notes:
Source: http://www.mos.org/evolution/fossils/fossilview.php?fid=33
So you do not have to “prove” there this is a “missing link”. What is this skull? Is it a human or is it an ape?
Is it a human/ape? If it is human then it appears you are going along with the species to species with no evidence of the “missing link”. Show me something that is part human and part horse and then you might convince me that you have true evolution and not adaptation.