Ever since Darwin, the scientific view has been descent with modification. We're all branches of the same phylogenetic tree, and many of those branches are very

Ever since Darwin, the scientific view has been descent with modification. We're all branches of the same phylogenetic tree, and many of those branches are very close to us, such as other mammals. We have topologically similar digestive, immune, circulatory, respiratory, nervous, and reproductive systems. While there are important differences between, say, the foods that a cat and a human can digest, the differences in how the overall system works are small compared to the similarities. Therefore, it's a reasonable starting guess that humans and closely-related animals would have many structural similarities in their mental experience, although with some differences. It tilts the balance towards needing evidence that other animals are not conscious, rather than needing evidence that they are. And when we look for evidence, we see intelligence even in branches as far away from us as crows and octopuses, suggesting that maybe consciousness (which we guess might correlate with intelligence) has deep roots in that tree, or else emerges independently quite easily.