Hello there! I’m a linguist and my research focuses mainly on Austronesian languages, but I was lucky enough to study under the amazing historical linguist Dr. Robert Blust while getting my MA, and I can actually answer this one!

The short and only answer is that language has not changed (in the way that this question is asking about) in any significant manner since it was first written down. Unfortunately, all we have to go on is written records. The study of the evolution of human language is essentially pseudo-science and guesswork. By the time the first languages were written, they were only limited by the writing system itself, and not by their ability to express thought. Take for example the Epic of Gilgamesh. This was written around 2100 b.c. and as you could read for yourself (translated of course), there are very complicated themes presented in this work. I’m sorry that the answer is so short, but the answer is simply that academic linguists have no idea how language evolved, and ever since it was written down, there have been no major “advances” in syntax, phonology, morphology, etc. There are changes (often staggering changes) in all of these categories in all languages over time, but there is no observable trend in language “advancing”. Be very wary of anyone who claims that they have reconstructed a “proto-world” language or claims to know how human language evolved. These claims are heavily doubted by every historical linguist I have ever met (although it is a fun topic to bring up at conferences), and I myself find all of these claims to be total bogus that do not meet the burden of proof.

Edit: I wanted to say that new words are constantly being invented, and old words constantly falling out of favor. However, this does not mean that we are able to express ourselves in a more complete manner than our ancestors. We even see this cross-linguistically today. A given language may have a single word for "my uncle's cousin's adopted younger niece". However, as you just saw, I could express the exact same idea in English by using a few more words. Languages are also very versatile. If it suddenly became useful or common for English speakers to mention their uncle's cousin's adopted younger niece, English speakers would likely invent a word for it. This relates to a major issue in the world that began during colonialism. Since that time, we have seen a staggering number of languages become "extinct" or "dormant", with one of the main driving factors being linguistic discrimination. I work with under-served linguistic communities in Indonesia, and many people see their language as "lesser" or "less educated" than standard Indonesian, Malay or English. This is simply not true, as all languages have the ability to express all concepts that another language can. Or at least, us linguists have not found any languages that are better or worse in this regard.