Oxnard C E, Franklin D
School of Anatomy and Human Biology, University of Western Australia, Crawley, W.A., Australia.
Folia Primatol (Basel). 2008;79(6):429-40. doi: 10.1159/000151356. Epub 2008 Aug 15.
Humans alone among primates lack a superficial head of the temporalis muscle, although a complete superficial muscle is present in 1% of humans and an incomplete one in 8%. Yet the temporal fascia of normal humans contains all the fascial sheets associated with that head even though it is absent. The implication is that humans have lost the superficial temporal muscle, that this is evident from the retention of the fascial sheets, and that the muscular variations represent situations where the muscle has persisted to some degree. Molecular factors in the head domain that are responsible for the development of the muscles of mastication (myosin heavy chain 16) are likewise different in humans than in all non-human species and seem to be responsible for the reduction of those muscles in humans. Could the loss of the superficial portion of the temporalis muscle be a component of this reduction? Could the uncommon muscular variations result from some slight persistence of the prior molecular situation? Could the persistence of the fascial sheets, even when the muscle is absent, be because the molecular factors responsible for connective tissues are not the same as those responsible for muscles? How much of all this can be visualised in the fossil record? Skeletal dimensions of the temporal fossa, partly related to the temporal muscle size, imply that it may be possible to determine in which fossils temporal muscle reduction has occurred. Likewise, surface features of the bone in modern humans without a superficial muscular component but with a strong complex fibrous element suggest that it might be possible to determine, in any fossil in which the surface preservation is good enough, how far back this situation may have persisted. It is already known that myosin heavy chain molecular dating suggests that the muscle reduction may have occurred about 2.4 million years ago.
在灵长类动物中，只有人类没有颞肌浅层头，不过1%的人类有完整的浅层肌肉，8%的人类有不完整的浅层肌肉。然而，正常人类的颞筋膜包含了与该头部相关的所有筋膜层，尽管该肌肉头部并不存在。这意味着人类已经失去了颞肌浅层，从筋膜层的保留可以明显看出这一点，而且肌肉变异代表了肌肉在某种程度上持续存在的情况。在头部区域中，负责咀嚼肌发育的分子因素（肌球蛋白重链16）在人类和所有非人类物种中也有所不同，似乎是导致人类这些肌肉退化的原因。颞肌浅层的丧失会是这种退化的一个组成部分吗？不常见的肌肉变异会是先前分子状况稍有残留的结果吗？即使肌肉不存在，筋膜层仍能保留，会是因为负责结缔组织的分子因素与负责肌肉的分子因素不同吗？所有这些在化石记录中能体现多少呢？颞窝的骨骼尺寸部分与颞肌大小有关，这意味着有可能确定哪些化石发生了颞肌退化。同样，现代人类中没有浅层肌肉成分但有强大复杂纤维成分的骨骼表面特征表明，在任何表面保存足够好的化石中，有可能确定这种情况已经持续了多久。已知肌球蛋白重链分子年代测定表明，肌肉退化可能发生在约240万年前。
