Final answer:
Tooth and jaw size in humans have reduced most significantly since the Neolithic period, which began around 10,000 yBP, coinciding with dietary changes from the adoption of agriculture.
Step-by-step explanation:
The size of tooth and jaw has reduced in humans since our ancient ancestors. One prime evidence of this change comes from Homo heidelbergensis, which lived approximately 600,000 years ago, represented by a specimen known as Mauer with a robust jaw and relatively small molars. By contrasting these early human ancestors with historical periods and nutritional changes, it can be deduced that the most significant reductions in tooth and jaw size likely occurred around the Neolithic Period, which began around 10,000 yBP (years Before Present). This period aligns best with the emergence of agricultural practices, leading to dietary changes that required less chewing and resulted in less robust dental features. Therefore, the correct answer to the student's question is b. 10,000 yBP.