There's a very fascinating (to me) species of monkey called the gelada. These Ethiopian monkeys (a relative of the baboon) first came to my attention because they have a key difference from other primate species. In most apes and monkeys a female's estrus state is indicated by a swelling and reddening of the buttocks.
For the gelada this is a problem. Geladas eat mostly grass, and they graze sitting down...meaning the female's buttocks are hidden in the grass all the time. So, instead, they changed signals to one the males could still see - a swelling and reddening of the breasts. Human females are either lying down, sitting, or standing - the buttocks don't make a great signal for us either...and this is probably why us girls have such splendid teats. As we evolved silent estrus the swelling of the breasts would have become a permanent sexual characteristic. So, the gelada drew my attention as, well, why we evolved the need for bras.
But now the gelada is in the news again. A researcher named Thore Bergman (University of Michigan) was studying geladas and noticed something creepy about their vocalizations. He kept thinking they were talking. It turns out that geladas use a pattern of vocalization and lip-smacking - that is very similar in frequency to human speech.
Could we have been missing something big here? Could it be that this relative of the baboon has, well, language? If so it would be a big discovery just for that - but it would also prove something else. It's long been the theory that language evolved from hunting signals.
Geladas don't hunt. Geladas are not only herbivores, but grazers. This clearly indicates that a sentient species could evolve from a grazing animal, something science fiction writers have played with (including me, with the ape-like, herbivorous tyrar - I swear, I wasn't consciously thinking of geladas).
Either way, I'm going to add the gelada to the list of potentially sentient or borderline sentient species on this planet - and very close to the top.