The hominid-bearing layers are 4.4 million years old, according to radioisotopic dating, geochemical examination of interbedded volcanic ashes, and biochronological factors.
ðə hˈɑːmɪnˌɪdbˈɛɹɪŋ lˈeɪɚz ɑːɹ fˈoːɹ.fˈoːɹ mˈɪliən jˈɪɹz ˈoʊld, ɐkˈoːɹdɪŋ tə ɹˌeɪdɪoʊˌaɪsətˈɑːpɪk dˈeɪɾɪŋ, dʒˌiːoʊkˈɛmɪkəl ɛɡzˌæmᵻnˈeɪʃən ʌv ˌɪntɚbˈɛdᵻd vɑːlkˈænɪk ˈæʃᵻz, ænd bˌaɪoʊkɹˌɑːnəlˈɑːdʒɪkəl fˈæktɚz.