GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 6J51IEC05NU56H04K

For more than fifty years, many evolutionary biologists posited that early fish such as Eusthenopteron developed limbs as a result of the need to drag themselves across short distances when their watery habitats dried up during periods of drought. However, new fossil evidence suggests that this hypothesis is incorrect. Fossilized remains of Acanthostega, a primitive fish, reveal that even though the animal had rudimentary limbs, it could not walk on land. Acanthostega lacked ankles, which means that its limbs couldn't support its weight; furthermore, its ribs were too short to prevent the organism's chest cavity from collapsing once the animal left water.