It is impossible to reconstruct a first language because language is too old, records of language do not go back far enough, and languages change too much, too quickly to reconstruct them far back enough. To be specific, it is believed that fully developed human languages probably existed at least 50,000 years ago, when there is first a lot of evidence for culture, such as paintings, ritual graves, and other meaningful symbolic artifacts. And language might go back as far as 250,000 years when anatomically modern humans first existed. But records of language only go back a little more than 5,000 years to Egyptian, Sumerian, and Hittite. And languages change so fast that if they're not written down they can change completely within a few hundred or, at most about ten thousand, years.