Our results show that many early hominins were generally smaller-bodied than previously thought, an outcome likely due to larger estimates in previous studies resulting from the use of large-bodied modern human reference samples. Current evidence indicates that modern human-like large size first appeared by at least 3–3.5 Ma in some Australopithecus afarensis individuals. Our results challenge an evolutionary model arguing that body size increased from Australopithecus to early Homo. Instead, we show that there is no reliable evidence that the body size of non-erectus early Homo differed from that of australopiths, and confirm that Homo erectus evolved larger average body size than earlier hominins.It turns out that all examples of the genus Homo were much smaller than modern humans until Homo Erectus came along and increased considerably in size.
Homo Erectus is also notable for having much more sexual dimorphism than modern humans, which is strongly suggestive of a gorilla-like harem social structure, with an alpha male having multiple female mates while being stalked by mate-less males seeking to displace him. This only abated sometime after Homo Erectus. While strong sexual dimorphism has been claimed to be significantly more pronounced in Neanderthals than in modern humans, the linked 1997 article in the New York Times reports a study that disputes that claim:
Dimorphism in primates is especially pronounced in gorillas and orangutans; the males are almost twice the size of females. Male chimpanzees are about 35 percent larger than females, which may also have been the size difference among Lucy and her kind, the early human ancestors known as australopithecines.A 2012 study concurs in the conclusion that there is weak evidence for strong sexual dimorphism in Neanderthals.
The celebrated Lucy, a fossil female from 3.2 million years ago, was a diminutive adult, 3 feet 7 inches tall and no more than 60 pounds. Another skeleton found in related African fossil beds, presumably that of a male, measured 5 feet 3 inches and 110 pounds. By contrast, modern humans are not only bigger, but their body-size dimorphism has declined. On the whole, men today are only about 15 to 20 percent heavier and 5 to 12 percent taller than women. . . .
LUCY (3.2 million years ago)
Lucy was a member of the Australopithecus afarensis branch of the human family tree, which flourished frm 4 million to 3 million years ago. Males were about 35 percent larger than females in her day, about the same as the dimorphism in chimpanzees today.
HEIDELBERG (300,000 years ago)
Homo heidelbergensis, a species ancestral to the Neanderthals, had males about 15 percent larger than females, on average, new findings in Spain show. This deviation is comparable to the male-female size differences of today.
Men are still 15 percent larger than women. However, scientists still don't know when dimorphism began to diminish from its early high level, which could indicate the timing of important shifts in early sexual and family behavior, as size became less of an advantage.
Another completely different methodology based upon digit ratios, which are a phenotypic marker of in utero testosterone exposure, favors polygnny in early apes, Ardipithecus, Neanderthals, and early modern humans, but not in Australopithecus ("Lucy").