I can tell you to begin with that the question is going to be impossible to answer for a couple of reasons.

The first is that there is no existing record of the inception of any language from scratch. The vast majority of evidence for Proto Indo European for example is lexical in nature rather than grammatical on a discourse scale because it is all extrapolated from a variety of different modern languages rather than having a primary source we can refer to. The best we’ve got is that PIE was very likely to be inflectional and probably had a default Subject-Object-Verb word order. Probably.

The second is because ‘complex’ in terms of comparing one language to another is kind of a vague notion. Within a single language we talk about certain things being ‘marked’ as a kind of way to distinguish between what is basic and what is more complex, but that doesn’t really work cross linguistically. For example, Japanese has a rather unusual alignment split from a typologically common nominative/accusative system to a less common direct/inverse system for their give/receive verbs. This is notoriously difficult for new learners of the language and could be considered ‘complex’, but native speakers have no issue with it and it is productive and usable in every day life. English speakers think it’s weird and complex; Japanese speakers think it’s just normal.

So that means that English uses a more simple system for expressing those same concepts right? Well…no. English has its own mess regarding primary and secondary objects and more vs. less marked structures. For example, why can I say both “I gave a present to him” and “I gave him a present.”? What is the difference on a functional level and which is more complex or basic? Why am I not saying “I gave a present him” or “I gave to him a present”? Why is that little ‘to’ so meaningful one way or the other? It can be difficult for non-native speakers to master, but a native speaker might not even think about it because it feels so simple that it isn’t even noticeable to them. Japanese doesn’t have this kind of distinction at all, so I’m that sense is less ‘complex.’

What I’m trying to explain, perhaps poorly, is that relative “complexity” of a language isn’t really a measurable thing, even when comparing a language to older versions of itself. Each linguistic feature is connected to all others within the language and can’t be singled out and analyzed in a vacuum.

Language change over time is driven by the opposing forces of distinctiveness vs ease. Humans are lazy, so they want to pronounce words as easily as they can and use as few words arranged as simply as possible as they can get away with, but at the same time they want to be unambiguously understood. ‘Complexity’ typically comes from a need to disambiguate or create contrast. This is one of the things that drives language change over time, so languages often trend towards simplicity via weird routes that create new complexity. “Today’s morphology is yesterday’s syntax” is a fun quote I remember hearing a long time ago. To give a current example, English has what is basically a syntactic future tense marker in the words “going to”, for example “I’m going to do this.” Because we’re lazy, we often slur this into “I’m gunna do this” and everyone understands us. But if “gunna” is just “going to” slurred together, why can’t you say “I’m gunna the store”? This is grammaticalization and language change happening in front of us as “gunna” is splitting off from “going to” and becoming its own distinct form. English currently has no future tense affix, but I have high hopes that hundreds of years from now we will be sticking the na- of “gunna” onto the beginnings of verbs to indicate future intent. Or maybe it will die off. No way to know right now.

I think I’ve gone way into left field about linguistics on a history sub, and my apologies for that. My only defense of myself is that historical linguistics is my favorite linguistics, and language change over time is fascinating. I would recommend reading more about the field of historical syntax and grammaticalization if this sounds interesting. Thom Givon’s ‘Syntax’ and ‘Functionalism and Grammar’ are old favorites of mine that touch on the subject while also providing a much broader context for understanding it. Some of the more focused literature might be a little obtuse without that context. If you’re interested in the history of change over time in English (and English is the most absurd mutt of a language so there’s a lot to this) ‘A History of the English Language’ by Albert Baugh is good and maybe more in line with the spirit of this sub. I am afraid I don’t have a recommendation for any literature that discusses the history of the discovery and development of our understanding of Proto Indo European from around the 1650’s when the idea of a common ancestor to a lot of languages first started appearing through modern times- if anyone does have such a recommendation I would be interested in reading it.