The human brain evolved over millions of years to become a relatively stable and predictable quantity. We know the rough adult weight of the brain, how it functions, and how abstract/reasoning intellect works vs. instinctive intellect. However, despite progress in teaching certain primates and even dolphins to follow orders and exhibit a form of understanding, humans have yet to conclusively show that non-human animals have intellect beyond instinctive and rote memorization.
One of the most important things about the human brain is its size; the human brain is 1:40 of the body mass, while animal brains are anywhere from 1:12 to 1:2789. The ratio of brain to body doesn't seem to have any real effect on intelligence; a mouse has the same ratio of 1:40 as a human, but is no more practically intelligent than a cat, which has a smaller ratio of 1:100. This creates an ethical problem; can we assume that animals -- at this stage of evolution -- are simply biological organisms with no feelings, intelligence, or soul? The subject of Animal Rights and Cruelty has dominated medical research testing, food preparation, and labor for over one hundred years, and shows no signs of stopping.
Proponents of Animal Rights often use anecdotal evidence to support their positions. While a beloved pet might show emotions familiar to the owner, a scientific examination of the pet will show it to be exactly the same as any other. There is not, at present, any objective reason to treat animals like humans; each person's individual and subjective opinion is valid but not conclusive in this sense. However, there is some scientific evidence that some animals -- dolphins and primates -- are slowly developing more abstract reasoning abilities. Studies are still being conducted to show improvement in reasoning and non-reactive intelligence.
Until such time as proof of human-like intelligence is found, there cannot be an objective obligation to protect species because of possible future evolution. However, since the Earth is a rapidly changing environment, we should always strive to conserve resources and limit unnecessary elimination of habitat and species. This is not an ethical obligation but a pragmatic one; we have only one planet, and without our natural resources, we will die out as easily as any other species.