Chapter 16, Problem 18A

Summary Introduction

Introduction : An organism is called bipedal when it can walk upright on two legs. One of the genus, hominins while becoming bipedal, developed a fully upright stance, shortened arms, restructured pelvic bones and foot bones.

Answer to Problem 18A

Correct answer :

The correct answer is option (c) divergence.

Explanation of Solution

Explanation/justification for the correct answer :

Option (c): A. afarensis were of the size as chimpanzee and had typical australopithecine skull and small brain. Their arms were long in proportion to their legs, and they had fingers and bones were more curved than those of modern humans. This trait indicated that A. afarensis could be arboreal.

However, this species had humanlike hip and knee joints and could walk upright. Later, it was concluded by Mary Leakey that australopithecine were bipedal from their fossilized footprints.

This shows that A. afarensis had divergent evolutionary pattern. In divergent evolution, two groups of the same species seem to evolve different traits within those groups to accommodate for differing environmental and social pressures.

Explanation for incorrect answer :

Option (a) convergence evolution is the process when organisms are not monophyletic, which means that they are not closely related, evolve similar traits independently in course of adaptation of similar environments or ecological niches.

Option (b) mosaic evolution occurs when different body parts or behaviors evolve at different rates.

Option (d) coevolution occurs between pairs of species or among groups of species when they interact with one another. In this, the process of reciprocal evolutionary change is seen.

Conclusion

Hence, A. afarensis was bipedal, but she exhibited apelike traits shows divergence evolution.