Abstract: Helical gold nanowires were experimentally found [1] and their atomic structure theoretically explained [2] very recently. Below certain critical radius, they were also found to spontaneously thin down to monatomic gold wires, before final breaking. Even though the measured structure of the helical nanowires can be well understood within ab-initio density functional theory (DFT) [2], at this level of description the simplest wires --the monatomic ones-- present a puzzle: DFT theoretical predictions of their stability-threshold-distances are far too short compared to experimental measurements [3]. Several attempts in order to explain what is the origin of these discrepancies seem to point out the role of light weight impurity atoms as the most likely factor. The presence of some impurity atoms, such as C or S, could distort easily the distances in the monatomic wires while remaining undetected to the experimental techniques [4-6]. An open question remains to be answered: what are the mechanisms that promote the formation of these hybrid nanowires?. In the present work, the formation of a hybrid monatomic nanowire is simulated via ab-initio DFT calculations. The starting point is a short helical gold nanowire with a single C atom approaching its surface. The simulation shows that at 400 K, the C atom strongly binds to the wire and dives-in very rapidly. In the process, the local structure undergoes noticeable changes leading to a $sp^3$ coordination of the carbon atom with four surrounding Au atoms. The Mulliken population analysis, computed for this configuration, reveals that the C atom has withdrawn electrons from the metallic environment, acquiring a charge of about 0.6 $e$. Upon further stretching, the helical wires would eventually break and the C atom becomes a key factor in the formation of the final hybrid monatomic wire. The final structure, before rupture, is a 9-atoms-long monatomic wire that connects to one of the tips via a the C atom, that became $sp^3$-coordinated with four Au neighbors. 1.\\, Y. Kondo and K. Takayanagi, Science, {\\bf 289}, 606, (2000).\\\\ 2.\\, E. Tosatti, S. Prestipino, S. Kostlmeier, A. Dal Corso, and F. D. Di Tolla, Science {\\bf 291}, 288, (2001).\\\\ 3.\\, J. A. Torres, E. Tosatti, A. Dal Corso, F. Ercolessi, J. Kohanoff, F. D. Di Tolla, and J.M. Soler, Surf. Sci. Lett. {\\bf 426}, L441 (1999).\\\\ 4.\\, Daniel Kr\\\"uger, Harald Fuchs, Roger Rousseau, Dominik Marx, and Michele Parrinello, Phys. Rev. Lett. {\\bf 89}, 186402 (2002). \\\\ 5.\\, Sergio B. Legoas, Douglas S. Galv\\\"ao, Varlei Rodrigues, and Daniel Ugarte, Phys. Rev. Lett. {\\bf 88}, 076105 (2002). \\\\ 6.\\, F. D. Novaes, A. J. R. da Silva, E. Z. da Silva, and A. Fazzio, Phys. Rev. Lett. {\\bf 90}, 036101, (2003).\\\\