So this isn't exactly an historical answer, so much as it is a linguistic one. I can't directly answer your question—though you might appreciate this answer by /u/bloodswan on the evolution of the English language—but I want to address the premise of it, because the question is using some faulty logic built on misconceptions, and I would like to clarify them for you.

Languages aren't designed to convey an accurate representation of the universe, nor is it evolving toward an asymptotic end-goal of perfection. They don’t necessarily get more complex as time goes by—check out, for example, /u/Bentresh’s sketch of Ancient Egyptian—nor is “more complex” a synonym for “better.” Languages reflect the culture of the people who speak them, and evolve to fill the needs of those cultures. A language can't be "more accurate" than another, it merely works in a different way than another language does - sometimes that difference is minimal, sometimes it's stark.

A classic example for this is the color spectrum, and how different languages categorize colors differently. You can ask an English speaker what color the sky is, and what color the background for the Zoom logo is (god, I hate that I'm using that as an example), and they'll say both are blue. But those are very different shades of blue! So are they the same color? Are these different shades of the same color, or separate colors altogether? Now, if you asked the same person what color the sky is and what color grass is, you'll get a much clearer answer - one is blue, the other green. But what if you asked a Japanese person? Well, the Japanese word ao refers to both blue and green, as if the two were actually different shades of the same color, so you might get a different answer (though there is also a more modern word that simply means green). There is a loooot of discourse regarding the relationship between linguistic relativity and perception of color, so I don't want to suggest that this proves something about how anyone perceives anything. I merely want to demonstrate that the difference in vocabulary exists, but it doesn't mean one is better or more accurate.

Since you suggest past humans might've had more limited vocabulary, a better example may come from the instance that set off the color discussion: Homer. In the 19th century, William Gladstone had a realization that the color blue is never mentioned in The Iliad or The Odyssey. The sea isn't "blue", according to Homer, it's "wine-dark." This may make it seem like the Ancient Greeks were strange for not using the word blue, but there's another explanation: blue doesn't occur that much in nature, particularly not in that ancient part of the world. And if it doesn't come up that much, there's not much need for a word for it. Blue became a lot more common in post-antiquity, when dying materials become easier - and as such, Ancient Greek eventually came to use a word meaning "blue" more often. (We can see similar patterns in other languages, and there is some discourse over how predictable the assortment and categorization of named colors in languages can be, but I can't really get into that.) This is really a simplification of a complex issue, so for more on that I’d point you to /u/KiwiHellenist’s excellent answer regarding the issue of using English terms to describe how non-Anglo cultures think about color:

The idea of a language ‘not having a word for blue’ assumes in advance that ‘blue’ is an objectively demarcated region of the colour spectrum. Which, of course, it isn’t. […] While it’s not strictly false to say ‘ancient Greek has no word for blue, therefore the Greeks had no word for sky colour’, it’s extremely misleading: you can just as easily say ‘English has no word for glaukos or lampros, therefore English has no word for sky colour’. It’s more true to say: ‘ancient Greek has multiple colour terms that overlap with the region of the colour palette that English calls ‘blue’.

The Greek language’s regard for color was organized around what its culture needed at the time. And that, in a way, is broadly why a language evolves: not to be more accurate, but more precise or specific, and fill the needs that it presently isn’t. But those linguistic needs only develop when the culture alongside it also does. One does not need a word for a concept to understand that concept, since we usually find another way to communicate it: Spanish has the word estrenar, for which there is no single word equivalent in English, but we still understand what it means “to wear something for the first time.” You wouldn’t fault ancient Hebrew for not having a word for what we call a computer, but when the language found a need for it in modern times, a word was (rather deliberately, I believe) found for it: mahh’shev, rooted in the Hebrew word for “think.” So a language being simpler or more complex doesn’t necessarily reflect how capable it is of communicating an idea, it just reflects the lengths to which the language will go to communicate it.

After all, Toki Pona is a language invented to be as minimalist as possible, yet it still can be used to translate things like The Lord’s Prayer and passages from the Bible. There is no such thing as “underdeveloped grammar” or a lacking of sufficient vocabulary, because language, uh, finds a way. They sometimes say that the past is a foreign country, and you can cross-apply to that to language as well: people spoke differently in different time periods, just as people speak differently in different parts of the world, but that difference in language isn’t a value judgement on the language. (As a FAQ Finder, I might as well commend to your attention the Language section of the FAQ if you want more discourse about language and history in general.)

And I would note that the idea that one language is better than another has been used for centuries to wipe out minority cultures when colonizers forced natives to learn the language of the empire and shamed or banned them from using their native language. And when those languages stop getting used, they get forgotten, and an element of their culture—and in turn, an entire way of looking at the world—gets wiped away, possibly forever.

If you want to look at language through the lens of them being complex, purpose-driven, and perfect, there’s a rabbit hole of invented languages to jump down. There’s a whole genre of conlangs called “engineered languages” which are languages designed to experiment with the possibilities of language or test some hypothesis about linguistics. Around the time of the Enlightenment (and even before then), there were a bunch of philosophers who believed that the Adamic language spoken in Biblical times before the confusion of tongues from the Tower of Babel was a perfect language wherein the nature and secrets of the universe where encoded within the rules of the language, such that words didn’t mean what they meant for arbitrary reasons (as they do now), but that there was a divine explanation for it, and they sought to recreate it. In the 20th century, Suzanne Elgin created Láadan to test if a language could more efficiently communicate female perspectives than natural languages do (supposing that most languages are male-centric). And most famously, Loglan and its successor Lojban were designed to make a language that is perfectly logical and has rules covering every situation one might encounter in communication, to remove any possible ambiguity and encouraging pure logical thinking. You will notice, though, that none of these languages really caught on.

Humans talk about wanting a perfectly efficient and effective language, but that’s not what they really want - they want a language that fits the needs of their society. And, for the most part, their languages already accomplish that, regardless of when or where they are.