We know, and have known for some time that the Pāli canon as we have it – and it is generally conceded to be our oldest source – cannot be taken back further than the last quarter of the first centure B.C.E., the date of the Alu-vihāra redaction, the earliest redaction that we can have some knowledge of, and that – for a critical history – it can serve, at the very most, only as a source for Buddhism of this period. But we also know that even this is problematic since, as Malalasekera has pointed out: “ … how far the Tipiṭaka and its commentary reduced to writing at Alu-vihāra resembled them as they have come down to us now, no one can say.” In fact, it is not until the time of the commentaries of Buddhaghosa, Dhammapāla, and others – that is to say, the fifth to sixth centuries C.E. – that we can know anything definite about the actual contents of this canon. We also know that there is no evidence to indicate that a canon existed prior to the Alu-vihāra redaction.
We know too that the earliest source we have in an Indian language other than Pāli – and this, according to Norman, is a translation – appears to be the Gāndhārī Dharmapada, the manuscript of which may date to the second century C.E. Of our Sanskirt sources, almost all from Central Asia, probably none is earlier than the fifth century, and the Gilgit Manuscripts, which appear to contain fragments of an Ekottarāgama, are still later. Our Chinese sources do not really begin until the second half of the second century, and it is, in fact, probably not until we arrive at the translations of the Madhyamāgama and the Ekottarāgama by Dharmanandin in the last quarter of the fourth century that we have the first datable sources which allow us to know – however imperfectly – the actual doctrinal content of at least some of the major divisions of the nikāya/āgama literature. It is from this period, then, from the end of the fourth century, that some of the doctrinal content of the Hināyāna canonical literature can finally be definitely dated and actually verified. Not before.
Gregory Schopen, “Two Problems in the History of Indian Buddhism The Layman/Monk Distinction and the Doctrines of the Transference of Merit” in Indian Monastic Buddhism Collected Papers on Textual, Inscriptional and Archaelogical Evidence (New Delhi, India: Motilal Banarsidass Publishers Private Limited, 2010), 23-25.