History of science topics tend to be underrepresented in NHD, so here are chemistry and biology topic suggestions for this year's theme! Some of these might be a bit of a stretch of the theme or might interpret it in an unusual way.
Something I find cool about this topic is that the 'legacy' bit explicitly requires discussing the impact in the present, so think about all of these with that in mind! Another thing to keep in mind is something that is specifically called out in this year's theme handbook: not everyone who "leads the way" shows leadership. The handbook actually specifically calls out scientists: "Does a scientist display leadership because he or she invents something that is historically significant? Not necessarily." The people I've listed here did great things in their field. The question to consider is whether they displayed leadership of others.
Chemistry
Kathleen Lonsdale, one of the first women elected as a Fellow of the Royal Society and a pioneer of x-ray crystallographyElsie Widdowson and nutrition
Dorothy Hodgkin, x-ray diffraction, worked out the structures of penicillin and vitamin B12
Anne McLaren and in vitro fertilization
Louis Pasteur and pasteurization or vaccination
Svante Arrhenius, founder of physical chemistry
Otto Hahn, father of nuclear chemistry
Fritz Haber, father of chemical warfare. This could make for an interesting argument about leadership.
Dmitri Mendeleev and the periodic table
