Final answer:
Homo heidelbergensis had a brain size averaging 1,200 to 1,400 cc, which is within the range of modern humans and larger than Homo erectus, indicating complex cognitive abilities.
Step-by-step explanation:
Relative to Homo erectus and Homo sapiens, the brain size in Homo heidelbergensis is similar to that of modern humans, averaging about 1,200 to 1,400 cubic centimeters. This brain size is slightly larger than the earlier Homo erectus, which had a progressive increase in brain size from about 550 cc to 1,250 cc over time. However, while Homo heidelbergensis had brain sizes within the range of modern humans, the structure of their brain may have been different, with some anatomical areas possibly emphasized differently compared to those in modern Homo sapiens.