British people of LT: Is there any grammatical mistakes in this text?

Hi there! I'd like to know if there's any grammatical mistakes in the following text:

The origin of language

The truth is nobody really knows how language first began. Did we all start talking at around the same time because of the manner in which our brains had begun to develop?

Although there is a lack of clear evidence, people have come up with various theories about the origins of language. One recent theory is that human beings have evolved in such a way that we are programmed for language from the moment of birth. In other words, language came about as a result of an evolutionary change in our brains at some stage.

Language may well be programmed into the brain but, despite this, people still need stimulus from others around them. From studies, we know that if children are isolated from human contact and have not learnt to construct sentences before they are ten, it is doubtful they will ever do so. This research shows, if nothing else, that language is a social activity, not something invented in isolation. 

• Some specific questions I have about it:

1. Does the sentence "From studies, we know that if children are isolated from human contact and have not learnt to construct sentences before they are ten, it is doubtful they will ever do so." make any sense? Or, should it be "From studies, we know that when children are isolated from human contact and have not learnt to construct sentences before they are ten, it is doubtful they will ever do so."? Or could it be either?
2. In this sentence: "This research shows, if nothing else, that language is a social activity, not something invented in isolation.", is there anything wrong with the use of the word "invented" in this context? 
3. Does the sentence: "Language may well be programmed into the brain but, despite this, people still need stimulus from others around them." sound unnatural?
4. Is the sentence "Did we all start talking at around the same time because of the manner in which our brains had begun to develop?" poorly written? Does it sound strange?

Thanks in advance for any help you're able to provide.

María

P.S. This is an English grammar exercise provided by Cambridge University Press, which I am going to attach below.

I'm not British, but I don't see any problems with the text. As to your specific concerns:

1) either

2) ok

3) ok

4) ok, but I prefer the more direct "Did we all start talking around the same time because our brains develop at roughly the same rate?"