Fill in the gaps to give a full meaning to the paragraph: And yet ______ the Mayas knew about the wheel, they never used

And yet, although the Mayas knew about the wheel, they never used it. Neither did they use metals other than copper. What is even more surprising is that they suddenly abandoned many of their cities and built new ones in the jungle. Some time around AD 900, Mayan civilization collapsed. By the year 1200, their last great capital, Chichen Itza, was deserted. Who were these strange people and the even stranger gods they worshipped? What brought about their sudden and mysterious collapse? Some writers have tried to prove that the Mayas had contact with visitors from space and even that they themselves came from another planet. Some people believe that their civilization came to an end because the Mayas never developed a proper resistance to local germs and diseases. All we really know is when the first Europeans appeared off their coast in 1517, this great and mysterious culture was only a memory.