Where Early Man was a hybrid of various ape or monkey forms and existing hominoid forms, Early Zeta was not a hybrid at all, but a transplant. The worlds we were placed upon had not sustained a life form complex enough to be considered a base for intelligence. The gap was too great. Early Zeta therefore was placed, full grown and conscious, on Zeta worlds. As this was to be our 3rd Density existence, Early Zeta was incarnated with forming entities prior to this transplantation, in fact at birth. Like the transplants who are coexisting with humans now on Earth, Early Zeta was aware of the purpose of the transfer. Thus transplanted, Early Zeta multiplied to the extent our worlds could support a population, much as humans have upon the Earth.
Primitive cultures among hominoids all bear some similarities, due to similarities of the body types. Nude and with little hair, the hominoid seeks protection from the elements. Weapons and tools fit the hand and arm, so are not that dissimilar among hominoid groups. Social structure, as anyone watching a group of monkeys can see, comes with the animal form. The dominant male, the polygamous male, the hunting bands, adoption of orphaned young - Early Zeta cultures developed along predictable lines, as do all early hominoid cultures.
Early Zeta differed from our present form primarily in brain size and mental capacities. Early Zeta was also stockier, shorter, had large flat feet and in appearance was something of a low brow, with the head jutting out the back rather than rounding. Our large eyes were an adaptation to our worlds, dim by your standards, so Early Zeta was a bit blind on his new home, not a problem as the only danger Early Zeta faced was from others of his kind. Our worlds do not contain carnivorous, nor even herbivorous animals - just plants and bugs.