There is a difference b/t that of and it is. 'That of' is possessive construction, in which 'that' stands for the thing being possessed. But 'it' is not part of any possessive construction, and therefore must stand for the noun that is the main focus of the preceding clause (which in this case is the early hominids' manner of walking).
同名同物指代
it 在这里指代the early hominids' manner of walking ,逻辑意思错误
通常it指代的是antecedent事物本身,而that指代的是同一类型,但是是另外一件。这里的it 就是这种错误,其指代的不是manner of walking而是hominids' manner of walking. 所以不合逻辑
"it" refers to the wrong subject
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