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Jejak Genom Leluhur Manusia

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JAKARTA, LEI – Makhluk berdiri tegak telah hidup dan menyebar di bumi lebih dari sejuta tahun lalu, mereka berjalan bungkuk dengan rahang bawah yang besar dan tonjolan tulang alis mata membentuk wajah seperti kera.

Para evolusionis abad 19 menyebut mereka merupakan nenek moyang manusia yang saat ini menguasai bumi. Hasil rekonstruksi dan perbandingan anatomi fosil tulang-belulang yang ditemukan di sejumlah lokasi di Eropa, Asia dan Afrika dijadikan dasar dari asumsi ini.

Misalnya sebagian tempurung tengkorak, gigi dan tulang paha atas yang ditemukan Eugene Dubois di Trinil, Ngawi pada 1890 dan dinamakan Pithecanthropus Erectus.

Sisa fosil dari zaman pleistosen tengah yang usianya diperkirakan sekitar 1,5 juta tahun itu menambah koleksi fosil manusia purba hasil perburuan para paleontolog evolusionis dalam mengungkap missing link yang mengaitkan garis keluarga kera ke homo sapiens, manusia modern yang hidup sekarang ini.

Namun hampir 1,5 abad berlalu, temuan spesimen fosil dari berbagai lokasi tersebut masih juga belum mampu menjelaskan asumsi-asumsi teori evolusi Darwin tentang leluhur bersama homo sapiens dan para hominid, meskipun teori ini telah menjadi bagian penting dari bangunan biologi.

Di sisi lain, ilmu genetik kini dalam perkembangan pesat dan tampaknya justru menambah daftar pertanyaan kepada teori terdahulu tentang pohon evolusi yang menjadi diagram percabangan spesies makhluk hidup berdasarkan anatomi semata.

Uji Genom

Sejak genom manusia selesai dipetakan satu dekade lalu, asal-usul dan jejak penyebaran manusia sudah bisa dilacak melalui gen.

Dengan uji DNA (Asam Deoksribo Nukleat) menggunakan kromosom Y, seorang pria zaman kini bisa melacak kakek moyangnya dari garis ayah, demikian juga dengan uji mitokondria DNA, seorang wanita bisa mencari tahu nenek moyangnya dari garis ibu.

Di setiap inti sel manusia terdapat 23 kromosom berpasangan yang masing-masing strukturnya berupa pilinan benang berisi informasi genetik berisi lebih dari tiga miliar huruf DNA dalam bentuk kelompok fosfat, gula dan nitrogen basa.

Manusia diperkirakan memiliki sekitar 30-35 ribu gen pembawa sifat per selnya yang terus diturunkan dari satu generasi ke generasi berikutnya melewati ribuan tahun tanpa banyak perubahan.

Riset DNA yang dilakukan terhadap lebih dari seribu orang dari berbagai etnik menyimpulkan moyang manusia modern mengacu pada seorang laki-laki dan seorang perempuan (Adam dan Hawa) yang berasal dari satu titik di Afrika Timur, yang jika dirunut berusia hingga 100-200 ribu tahun.

Benua Afrika menjadi tempat orang-orang pembawa gen paling tua, sedangkan orang-orang di luar Afrika seperti Eropa, Asia, Amerika, termasuk penduduk asli Australia dan Papua, hanya mengarah pada usia gen setua 50-70 ribu tahun lalu.

Dari sana disimpulkan bahwa pada periode tersebut sejumlah kelompok moyang di Afrika Timur kemudian bermigrasi secara bertahap ke luar dari benua tersebut, melalui jembatan semenanjung Arabia dan menyebar ke seluruh dunia.

Berdasarkan analisis terhadap ribuan sampel DNA yang dilakukan oleh tim pakar genetik yang dipimpin Spencer Wells, migrasi leluhur bersama (common ancestors) orang Eurasia (out of Africa) ini terbagi dalam sejumlah cabang.

Cabang pertama dalam peta penyebaran manusia, diketahui moyang dari Afrika timur itu menuju ke utara di Timur Tengah (45 ribu tahun lalu) menetap lama dan kemudian generasi-generasi berikutnya ada yang bermigrasi lagi ke Afrika Utara dan kemudian sebagian mereka ke Eropa Selatan. Proses migrasi ini terjadi bertahap dalam ribuan tahun.

Cabang lainnya ada yang menuju ke Asia Tengah 40 ribu tahun lalu, yang dari sana, keturunan berikutnya mulai merintis jalan ke utara.

Mereka ini ada yang berbelok ke barat (35 ribu tahun lalu) bercampur dengan kelompok dari Timur Tengah menjadi leluhur orang-orang Eropa, ada yang ke barat-laut menuju Rusia dan berbelok menyebar di kawasan Skandinavia.

Perintis Asia Tengah ini dalam atlas genetik kemudian menyebar ke Asia Selatan dan Indo-China lalu berbelok lagi ke utara menyebar di Asia Timur. (antara)

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9 Comments

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