Picking up where I last posted on the previous page.
How to Save the Natural World on Which We Depend
PAS-PASS Workshop, Casina Pio IV, 27 February-1 March 2017
On our 4.54 billion year old planet, life is perhaps as much as 3.7 billion years old, photosynthesis and multi-cellularity dozens of times independently around 3.0 billion years old, and the emergence of plants, animals, and fungi onto land, by at least the Ordovician period, perhaps 480 million years ago, forests appearing around 370 million years ago, and the origin of modern groups such as mammals, birds, reptiles, and land plants subsequently. The geological record shows that there have been five major extinction-events in the past, the first of them about 542 million years ago, and suggests that 99% of the species that ever lived (5 billion of them?) have become extinct. The last major extinction event occurred about 66 million years ago, at the end of the Cretaceous Period, and, in general, the number of species on earth and the complexity of their communities has increased steadily until near the present.
Over the past 66 million years, the number of species has grown to an estimated 10-14 million kinds of eukaryotic organisms (those with complex cells) and an unknown but very large number of prokaryotic organisms (archea and bacteria). The first of our close relatives that we have discovered in the fossil record are about 2.7 million years old, and like all of our earlier relatives, they occurred in Africa. Homo erectus, the species most closely similar to us, migrated out of Africa via the Middle East starting about two million years ago, and was followed by Neanderthals, Denisovans, and ultimately, **about 60,000 years ago, by our species, Homo sapiens. **Our ancestors soon spread out of Eurasia and by something like 12,000 years ago, had occupied all of the continents. By about 30,000 years ago they had conquered and killed all other forms of humans that had reached the Northern Hemisphere earlier.
For some tens of thousands of years after they reached Eurasia, humans lived as hunter gatherers. During that time they certainly began to make artistic works, weapons, musical instruments, and the like, but since they kept moving in search of food, carrying their babies with them, there was not much chance to develop what we consider civilization today. There may have been sporadic cultivation of small patches of crop plants earlier, but our ancestors turned to cultivation much more extensively about 12,000 years ago. By 10,000 years ago crops, along with domestic animals, provided a major source of storable food, one that could see them through droughts, winters, and other unfavorable times, and the numbers of people that could live together in a village, town, or city was greatly increased, allowing all aspects of civilization to develop much more fully in these centers.
At the time crops became important elements for human survival, 10,000 years ago, the entire world population is estimated to have been about one million people, with about 100,000 in Europe. Written language was developed about 5,000 years ago as distinctive civilizations were appearing in different parts of the world. Human populations began to grow rapidly and overwhelmed the capacity of many natural systems through cultivating crops and grazing. It is estimated that at the time of Christ, there may have been 300 million people globally; now there are 7.3 billion. Some 11% of the world world’s ice-free land surface have been converted to crop agriculture, another 20% to grazing, most of it unsustainable, on natural grasslands. It is obvious that many of the kinds of organisms that occurred 10,000 years ago have already gone extinct, and that we are dealing with a reduced set of the organisms that existed when agriculture was first adopted by our ancestors. What percentage would have been lost in this period is unknown, but on islands it seems to have been a majority, and on continents a large percentage also. Our civilization and our numbers grew in a relatively stable period of climate following the last expansion of continental ice sheets about 26,500 years ago, and we are now profoundly damaging the conditions under which our numbers have increased from about 1 million to about 7.3 billion people, with a net of 250,000 extra people every day.
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pas.va/content/accademia/en/events/2017/extinction.html
Here is another great website I encourage people to read:
actionbioscience.org/evolution/johanson.html#primer:)
