Under a covering of mica rich sand and sediment there are a surprising number of footprints – in what had been a field near Tucson in Arizona – see http://westerndigs.org/oldest-human-footprints-in-the-southwest-discover… … and they go back 2500 years. They appear to belong to a farming group, adults and children, and even their dogs, and they were perfectly preserved as a result of being covered by a layer of sediment brought down stream by a flash flood (a small creek is situated nearby). C14 samples came up with a date between 2500 and 3000 years ago – in the first millennium BC. Irrigation ditches have been found and little planting depressions.
Staying on a similar theme at www.spiegel.de/international/world/new-sahara-research-the-lakes-of-ouni… … we have an oases with lakes of water bang in the middle of the Sahara desert, a relic of a much bigger lake that existed in what is now northern Chad in the first half of the Holocene. Sediment cores have been extracted from the lake and these are said to go back 10,000 years and show the desertification of the region was gradual – and not rapid. The cores also show emphatically that there was a lot of water in the Sahara over 5000 years ago. A watery oasis still exists – these are the lakes of Ounianga. A vast reservoir of underground fossil water lies beneath the surface and feeds the lakes. If that underground source did not exist the lake would have dried up long ago. See also http://www.archive.aramcoworld.com/issue/201403/last.lakes.of.the.green….
This link is also about the oases of northern Chad and this time Stefan Kropelin in accompanied by Peter de Menocal, a palaeo-climate scientist of note. His idea was that the monsoon track shifted and the Sahara turned into desert relatively quickly. Kropelin thinks it took a long time. Take your pick. Apparently, we are told there may have been hundreds of wet phases of climate in the Sahara, and equally hundreds of dry phases of climate. That is the theory. Obviously, a lot of work to be done on what was going on in the Sahara prior to Holocene.
(the last link doesn't appear to work. Instead, go to http://aramcoworld.com and click on the menu and then click on archive. Then choose the journal of choice. In this case it is 2014 issue 3. Once into that click on and it will be the third article to appear. Worth playing around with for a while – interesting site too).