Mackay was a scientist.
Background
He formulated a theory on mutations, which states a proportional relation between the time an animal is exposed to radiation and the size of the animal. If the animal is exposed to long-term radiation, the larger that animal will grow. This theory is known to the scientists of Vault 101.
In 2241, Anne Palmer writes in her official report to the overseer that she believes evidence existed to prove this theory. During their survey of the ruins of Springvale, her team encountered giant ants. A battle ensued in which some of the ants were killed. The physical proof of these giant insects convinced team leader Palmer that she had proof of Mackay's theory. She had some of the ant corpses removed to Vault 101 for study. In the overseer's terminal, there is a photo of a giant ant the team photographed for their survey.[1]
Notes
The Lone Wanderer learns of the theory from reading Anne Palmer's official report in the overseer's terminal.