Louis Jean-Baptiste Alphonse Bachelier March 11, 1870 – April 28, 1946) was a French mathematician at the turn of the 20th century. He is credited with being the first person to model the stochastic process now called Brownian motion, as part of his PhD thesis The Theory of Speculation (Théorie de la spéculation, published 1900)
Bachelier's Doctoral thesis, which introduced for the first time a mathematical model of Brownian motion and its use for valuing stock options, is historically the first paper to use advanced mathematics in the study of finance. Thus, Bachelier is considered as the forefather of mathematical finance and a pioneer in the study of stochastic processes. (Source: Wikipedia).
On April 21,2019 the CME clearing house switched the options pricing and valuation model to Bachelier to accommodate negative prices in the underlying futures and allow for listing of option contracts with negative strikes.
And, in August the CME switched back: “Further to Clearing Advisory 20-171 dated April 21, 2020, CME Clearing will revert its options pricing and valuation methodology, currently based on the Bachelier model, effective for trade date on Monday, August 31, 2020. Products will transition from Bachelier back to Whaley or Black 76, depending on the product.”
For more insight into the Bachelier option modeling here is a link to paper On the Option Pricing Formula Based on the Bachelier Model by Satoshi Terakado, September 2019