Paleoanthropologists believe that the predominant diet of our earliest omnivorous ancestors consisted mostly of

roots, tubers and fruits

Explanation:

Although omnivores are both plant and meat eaters, paleoanthropologist studied that the earliest omnivores ancestors (such as humans) consumed diets that constitute roots, tubers and fruits. Studies show that not until about 2.6 million years ago was meat largely incorporated into the diet of humans. The study of the morphology of the tooth also showed these evidences. They also found no evidence of butchered bones. In a bid to research into the earliest omnivores, it was discovered that there was no decrease in the size of the gut and tooth and also brain size increase, which are features linked to the consumption of meat.

productive most commonly means: achieving or producing a significant amount or result.

explanation:

productive can also mean:

these depend on how the word productive is used. most likely productive will mean   achieving or producing a significant amount or result.