[14] Since early nomadic hunter-gatherers were prehistoric — with no written histories — we only know them from the mysterious clues they left us. And none are more mysterious than their huge stone monuments found all over Europe. This is from the "megalithic" age — characterized, literally, by big stones.

[15, Stonehenge, c. 2500 BC, Amesbury, England] At Stonehenge, in England, huge carved stones — some over 20 feet tall and weighing 25 tons — stand in a circle. The stones were erected with Stone Age technology, before the advent of metal tools. It's hard to imagine, as we look at the museum's crude artifacts and replica thatched-hut hamlet, how these humble early people managed it.

[16, Stonehenge Visitor Centre] Huge stones like this replica were quarried, carved, and then moved for many miles — some of them from as far away as Wales, 200 miles to the west. They barged them down rivers; they may have rolled them on logs like this — nobody knows for sure.

[17, demonstration at Stonehenge Visitor Centre] Marveling at these stones, we ponder the purpose of these stone circles — perhaps sacred centers of ritual and worship. We know they functioned as celestial calendars. As this 360-degree theater demonstrates, the structure is aligned with the heavens — marking both the longest and the shortest days of the year. Four thousand years ago locals could tell when to plant — and when to party — according to where the sun rose and where the sun set. And even today, as the sun rises on the longest day of the year in just the right spot, it casts a powerful spell of wonder.

[18, Avebury, England] Nearby, at Avebury, visitors wander through a cohesive ensemble of ditches, mounds, and megaliths — the work of people clearly on a mission. The huge circle — while now cut in two by a busy road and so big it contains a village — retains its allure.

[19, megaliths, Orkney, Scotland; Scorhill Stone Circle, Dartmoor, England; Castlerigg Stone Circle, Keswick, England] Hundreds of these Stone Age calendars are scattered over the isle of Britain — each built by people working together — people doing more than merely surviving. My favorites are the ones that are tranquil, all alone in a field, where you can reflect on the mysteries of who built them and why.