Final answer:
The term 'inter-membral index' does not correspond to a commonly used scientific term and might be a confusion with another concept. The most likely related topic in the provided context is the 'Encephalization Quotient' (EQ), which is a measurement used to compare brain size relative to body size across different species, with humans having an EQ between 6.0 to 7.0.
Step-by-step explanation:
The inter-membral index is a term that seems to be a typographical error or a misinterpretation of a scientific term, as it is not standard. However, for the provided information regarding brain size and cognitive abilities across different hominin species, the relevant concept would be the 'Encephalization Quotient' (EQ). The EQ is a measure of brain size relative to body size and is used in evolutionary biology to make comparisons across species. Modern humans, Homo sapiens, have an EQ of approximately 6.0-7.0, which suggests that our brains are six to seven times larger than what would be expected for an animal of our body size. By comparison, Homo erectus has an EQ of 4.0, indicating a smaller, but still enlarged brain compared to body size. In the case of australopithecines, their EQ falls between 2.5 to 3.0. It is important to understand that the 'inter-membral index' is likely not the correct terminology and may refer to something else entirely such as limb proportions, which is distinct from brain size or EQ.