Lecture 21 The Hominid Fossil Record
1. The hominid fossil record is a busy, complex record that expands each year with new discoveries. We will cover some of the "hominid highlights" here but I encourage you to keep on the lookout for new discoveries, which occur frequently. We can begin by looking at the famous Laetoli footprints (found in Tanzania), which are 3.6 million years old and were made in a dusting of volcanic ashfall. They are actually two sets of footprints, set among animal tracks of various kinds. Who was that, walking upright across the plain, 3.6 million years ago? What kind of fossil record do we have? Let's take a look.
2. First, a quick review of the Cenozoic time scale. Remember that the Cenozoic Era is divided into numerous epochs. We have already taken a close look at the Eocene and the Miocene Epochs, but the Hominid fossil record really is a story that develops in very late Miocene time, and continues right through the Pliocene/Pleistocene/and Holocene Epochs. So, our story really begins at the close of the Miocene Epoch.
3. If we examine the Miocene-Pleistocene Epochs in detail, we see that the Miocene-Pliocene boundary is at about 5.3 million years ago, and the Pliocene-Pleistocene boundary is at about 2.6 million years ago. We can also see from the range lines of the fossil hominids, that much of diversity occurs in the late Pliocene and the Pleistocene, but that the record gets started near the Miocene/Pliocene boundary.
4. Let's also consider the biological classification of the groups we will be talking about. First of all, in the Primate Order there is a superfamily called the Hominoidea, which contains 3 distinct families-the great apes (chimpanzees, gorillas, orangutans); the lesser apes (gibbons, siamangs), and the Hominid family-which consists of modern humans and their extinct ancestors .
5. The entire Hominoidea superfamily has a fossil record that begins in the Eocene, and an ancestor of the Hominoid line is the fruit-eating, arboreal Egyptopithicus. We will concern ourselves here with the record further forward in time-that of the Hominid family, one family within the Hominoid superfamily.
6. In the brief amount of time we have, we can't examine all these hominid fossils, but we can take a look at 3 that are particularly significant: Ardipithecus ramadus (known as "Ardi" ), Australopithecus afarensis (known as "Lucy"), and Homo erectus. Ardi and Lucy are separated by about a million years in time, and Homo erectus is younger still, another million and a bit ahead in time. All these species arose in Africa, and the amazing fossil record is mostly (although not entirely) in east Africa, in the Afar triangle and East African Rift area.
7. Tectonically, east Africa is very active. It contains a triple junction, where 3 spreading (rifting) areas meet-the Red Sea, the Gulf of Aden, and the East African Rift. This triple junction area is the Afar triangle, which is a deepening graben that is subsiding, and collecting immense loads of sediment through the Cenozoic. The African rift valley lakes along the eastern portion of Africa mark the line of the rift there. These lakes are very deep, because they are downdropping grabens, and they also have wonderful fossil records.
8. Ardipithecus ramadus (known as Ardi) is the closest fossil skeleton known so far to the ancestor that split off from the other Hominoidea. Ardi is latest Miocene-about 5.3 million years in age, and was discovered in the Afar triangle. Ardi walked upright, but Ardi still has a grasping big toe for climbing. The associated fossils with Ardi show that Ardi was living in a forested area, not on the grassy plains. Watch this very short video (about a minute and a half) discussing the importance of the footbones in this skeleton.
9.Ardi's feet!
http://www.youtube.com/watch?v=whtQ8iUXwqQ&feature=relmfu
10. By Pliocene time, grasslands and savanna environments had continued to spread, and grazing mammals continued to diversify. Australopithecus afarensis "Lucy" dates from this time-about 3.6 million years ago-this is the hominid that made the footprints at Laetoli. Lucy and her kind walked upright, and if you examine the feet, you will see that the grasping toe is no longer a feature-the foot is a recognizable "walking " foot that we are familiar with.
Watch/Listen to this short video narrated by Donald Johanson, the discoverer of "Lucy" about the hominid fossil record, human evolution, and "Lucy"
http://www.youtube.com/watch?v=HphLBNGCBNk
11. If we leap forward in time again, to more than a million years ahead of Lucy, we encounter Homo erectus, probably the most successful of all the hominids. Homo erectus as a species had a very long duration-almost all of the Pleistocene-close to 2 million years. Homo erectus arose in Africa, and a population of them migrated out of Africa to Eurasia beginning about 1.8 million years ago, and had continuing migrations through about 1.2 million years ago.
Homo erectus was a tall, upright walking, tool using hominid that ranged throughout Africa and Eurasia and into the Indonesian Archipelago.
The climates of the Pleistocene were challenging to say the least-about 120 glacial and interglacial stages occurred during this time, and Homo erectus made it through most of them.
12. The migration of Homo erectus is only one of several migrations out of Africa. About 800,000 years ago, another species that arose in Africa, Homo heidelbergensis, also migrated out of Africa and spread to western Europe and the middle East. Again about 60,000 years ago, Homo sapiens (Cromagnon) also arose in Africa, and a population moved out of Africa, this time in a migration that extended outward to all the continents by about 12,000 years ago.
13. The Hominid Family Tree has many branches. Most interesting of all, just 30,000 years ago (during the most recent glacial event), 3 varieties of closely related hominids roamed the landscape, and genetically intermixed (neanderthals, and a newly discovered group called the Denisovans (discovered in Siberia) and Cromagnon (modern humans). Since the most recent 30,000 years, only one hominid, Homo sapiens, has survived.
14. A mysterious, small descendent of Homo erectus appears to have survived in the Indonesian archipelago up until the beginning of the Holocene, so that means that the Pleistocene had at least 4 hominids co-occurring.
15. The migration of modern humans (Homo sapiens) began about 60,000 years ago. Until a few years ago, two competing hypotheses were proposed to explain the modern distrubution of humans-one was the "out of Africa" model, one proposed multiple spreading centers. However, recent DNA analysis, as well as research coming from the Human Genome Project, has demonstrated the the "out of Africa" model is best supported by both the fossil record and genetics.
16. Next Lecture-
How did the Ice Ages begin? We will look at the transition to the "Ice House", and explore the Pleistocene record.