H. habilis is one of the oldest species in the genus Homo . Nevertheless, evidence suggests that in some ways, it was quite similar to species in the genus Australopithecus , especially in aspects of the postcranial skeleton and the small size of its brain. Taking into account body size and shape, locomotion, the masticatory system, and brain size, some scientists suggest that H. habilis had an adaptive strategy more similar to australopiths than to modern humans and should be placed within the genus Australopithecus. Whether or not this is a valid suggestion depends upon how a genus is defined. Scientists disagree as to whether phylogeny (evolutionary relationships) should be given priority over adaptive strategies when defining a genus, or vice versa, a distinction that is not easy to make, especially when dealing with fossil specimens. Currently, H. habilis is placed within the genus Homo because it shares derived traits with other members of the genus to the exclusion of the australopiths.
Each skeleton muscle contains a vast number of muscle fibres (cells), each is 10-100 µm thick and up to 15 cm long. The sercolemma, or the contraction mechanism, of a single muscle fibre, is defined as the zone between two delicate intermediate striations (Z), each is connected to isotropic (I) bands, which move (in reaction to depolarization in a neuromuscular junction) one toward the other between wider anisotropic fibres (A) and reach the middle light zone (H). In simple words, the contraction of a singe muscle fibre occurs when I bands move towards the central H zone, while isolated by the A fibres. When they move, each I band pulls the Z “plate” to which it is connected, leading to a contraction in the muscle.
Biology Label Printouts
Whales Physical Sciences: K-12
Art and Artists
Label Me! Printouts