Summary :p

It has been difficult to investigate the origins of language. First of all, it is invisible in archeological records. The oldest know written word dates back only to 5000 years ago. What is more, scholars avoid the problem of the origins of language. It is a foregone conclusion that the development of language had influenced humans’ behavior – still, it is unknown how do those factors exactly intermingle.

It has been difficult to investigate the origins of language. - czemu Present Perfect? od jakiego czasu trudne?
The oldest know<-brak litery written word dates back TO ONLY 5000 years ago. What is more, scholars avoid tu sie przyda PresPerfCont lub Simp the problem of the origins of language. It is a foregone conclusion that the development of language had influenced <-czemu past perfect? simple past HUMAN behavior, ale czy nie chodzilo Ci o 'the behavior of our przedkowie?' – still, it is unknown how do pytanie w mowie zaleznej polega na czym? those factors exactly intermingle. Zastanawia mnie zmiana czasu na present, skoro piszesz o poczatkach jezyka to ostatnie zdanie nie powinno miec charakteru uogolnienia.