近日，国际著名杂志《国家科学院院刊》PNAS刊登了美国芝加哥大学的研究人员的最新研究成果“Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes。”，由于非洲肺鱼可以用腹鳍行走并用肺呼吸，因此可能是进化环节中的一个关键环节。
该研究显示，非洲肺鱼可能是早于四足动物或者与之同一时期的鱼类。四足动物由泥盆纪的肉鳍鱼进化而来，是最早占领陆地的脊椎动物。生物学家梅利娜·黑尔指出，研究人员在寻找“失去的环节”时发现有动物能够交替使用四肢，这说明“人类已经发现形态完全不同的水生动物，虽然它们的行走方式与四足动物非常相似，但它们并非四足动物”，肺鱼就属于这种情况。
黑尔相信，非洲肺鱼“与从水中进化到陆地上的动物有着非常密切的关系”。她指出，几乎所有那个时期的鱼类都灭绝了。非洲肺鱼的特征是能够运用腹鳍抬起身体并向前行走。在行走过程中，它还能够和四足动物一样交替使用两个后肢。
此项研究的作者指出，虽然过去人们已对非洲肺鱼的形态有所了解，但是尚无研究表明它能够利用腹鳍行走。他们在一个水箱中复制了非洲肺鱼的生活环境，并利用摄像头从多个角度记录下它的活动，通过这种方式对它的移动方式和腹鳍的活动进行全面研究。
结果研究人员发现，非洲肺鱼完全能够利用腹鳍支撑身体并向前行走。他们还发现，非洲肺鱼之所以能够用纤细的两个后肢行走是因为水中的重力较小。此外，它能通过用肺呼吸提高自身浮力，从而减轻“腿”受到的压力。研究人员指出，必须利用活体动物进行研究，因为通过化石只能确定腹鳍的形态而非功用。此外，他们还指出，生物从水中到陆地的进化过程可能早在四足动物之前就已经发生。（生物谷Bioon.com）
doi:10.1073/pnas.1118669109
PMC:
PMID:
Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes
King, Heather M.; Shubin, Neil H.; Coates, Michael I.; Hale, Melina E.
Tetrapods evolved from sarcopterygian fishes in the Devonian and were the first vertebrates to colonize land. The locomotor component of this transition can be divided into four major events: terrestriality, the origins of digited limbs, solid substrate-based locomotion, and alternating gaits that use pelvic appendages as major propulsors. As the sister group to tetrapods, lungfish are a morphologically and phylogenetically relevant sarcopterygian taxon for understanding the order in which these events occurred. We found that a species of African lungfish (Protopterus annectens) uses a range of pelvic fin-driven, tetrapod-like gaits, including walking and bounding, in an aquatic environment, despite having a derived limb endoskeleton and primitively small, muscularly supported pelvis. Surprisingly, given these morphological traits, P. annectens also lifts its body clear of the substrate using its pelvic fins, an ability thought to be a tetrapod innovation. Our findings suggest that some fundamental features of tetrapod locomotion, including pelvic limb gait patterns and substrate association, probably arose in sarcopterygians before the origin of digited limbs or terrestriality. It follows that the attribution of some of the nondigited Devonian fossil trackways to limbed tetrapods may need to be revisited.
