Some of the most unusual effects of Hellenization can be seen in Afghanistan and India, in the region of the relatively late-arising Greco-Bactrian Kingdom (250 BC-125 BC) in modern Afghanistan, Pakistan, and Tajikistan and the Greco-Indian Kingdom (180 BC - 10 CE) in modern Afghanistan and India. There on the newly formed Silk Road Greek culture apparently hybridized with Indian, and especially Buddhist culture. The resulting syncretism known as Greco-Buddhism heavily influenced the development of Buddhism and created a culture of Greco-Buddhist art. These Greco-Buddhist kingdoms sent some of the first Buddhist missionaries to China, Sri Lanka, and the Mediterranean (Greco-Buddhist monasticism). The first figural portrayals of the Buddha, previously avoided by Buddhists, appeared at this time; they were modeled on Greek statues of Apollo. Several Buddhist traditions may have been influenced by the ancient Greek religion: the concept of Boddhisatvas is reminiscent of Greek divine heroes, and some Mahayana ceremonial practices (burning incense, gifts of flowers, and food placed on altars) are similar to those practiced by the ancient Greeks. One Greek king, Menander I, probably became Buddhist, and was immortalized in Buddhist literature as 'Milinda'. The process of Hellenization extended to the sciences, where ideas from Greek astronomy filtered eastward and had profoundly influenced Indian astronomy by the early centuries AD. For example, Greek astronomical instruments dating to the 3rd century BC were found in the Greco-Bactrian city of Ai Khanoum in modern-day Afghanistan while the Greek concept of a spherical earth surrounded by the spheres of planets was adopted in India and eventually supplanted the long-standing Indian cosmological belief of a flat and circular earth. The Yavanajataka and Paulisa Siddhanta texts in particular show Greek influence. 
What cultural phenomenon led to the existence of a Greco-Bactrian synthesis?
The Silk Road