mfagan

Paleo Essay

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Humans in the Fossil Record

argh so much data...

http://en.wikipedia.org/wiki/Human_evolution

http://en.wikipedia.org/wiki/List_of_human_fossils

http://www.talkorigins.org/faqs/homs/

http://spreadsheets.google.com/ccc?key=o11460430574250214230.4830567682205576433

We study paleontology to learn about the history of our world, because understanding history is the key to understanding the future, and because we are curious. Nowhere do these qualities apply more strongly than in our own history as a species. What we learn can tell is profound things about who we are.

Unfortunately, finding humans in the fossil record is difficult, for several reasons. We are terrestrial, and terrestrial fossils occur much more rarely than do aquatic ones. Beyond this, much of our early habitat, as far as we can tell, was poorly suited for the processes of fossilization. Our divergence from our closest living relatives - the chimpanzees - occurred only about 4-6 million years ago {get data!}, and so our fossils occur only in a vary narrow span of time.

Types of Fossils

{not the first section}

bones
footprints
art and tools (?)

Humans in the Fossil Record

Introduction

What are humans in the fossil record?

When looking for human fossils, it is first important to define exactly what constitutes a human fossil. For the purposes of this paper, we will regard human fossils with a fairly broad definition. Any fossils from – roughly speaking – the time in which we separated from our closest living relatives (the chimpanzee, and to a lesser extent, the gorilla), will be regarded as “human” fossils.

Anatomical and genetic tools tell us that we fit into the great apes, the family Hominidae. It is best to see our classification in greater context, as follows:

Phylum: Chordata

Class: Mammalia

Order: Primates

Family: Hominidae

Genus: Homo

Species: sapiens

Subspecies: sapiens

Thus, we are Homo sapiens sapiens, the only extant species of the genus Homo. With the aforementioned chimpanzee (two species), gorilla, and also the orangutan, we form the five extant Hominids.

Finding Fossils

Finding human fossils can be a very difficult task. To begin with, as we have learned, the processes which create good fossils are considerably more likely to occur in aquatic, rather than terrestrial environments. Unfortunately (so to speak), humans are a terrestrial species, and to the best of our knowledge, Hominids have been exclusively predominantly terrestrial. There is a theory that there were one or more semi-aquatic phases in human evolution, however it is not widely accepted and beyond the scope of this paper.

Within terrestrial environments, we have not always lived in environments particularly conducive to fossilization. Probably the most significant reason for the lack of human fossils is a lack of rapid burial – like other large animals that die (such as an antelope in Africa), between microscopic bacteria and large mammal scavengers, little of an animal remains for very long.

“Most hominid fossils have been found near to ancient lakes and rivers, partly because our ancestors, like most mammals, were highly dependent on water; and partly because these provide the best depositional environments where fossil formation is favored” p29

“Finding hominid fossils is always a fortuitous undertaking. Most commonly (and still requiring considerable luck), one or two pieces of an individual are found. Only when extraordinary preservation conditions allow, are more complete individuals recovered. For example, there is the famous “Lucy” find, comprising 40% of a skeleton, from the Hadar site in Ethiopia.” http://www.wadsworth.com/anthropology_d/special_features/ext/latest_dirt/southafrica.html

“Unlike most other fossil types, however, there are so few good hominid fossils that each new discovery can have profound importance for reconstructing the course of human evolution.” http://www.cns.uni.edu/~groves/LabExercise13.pdf

pretty much just bones found, especially skulls, jaws, teeth...

The Great Rift Valley

Geology comes into play in a big way for some of our most important fossil finds. The Great Rift Valley is a massive geologic feature, running from Lebanon in the north, two-thirds of the way to the southern end of Africa, marking the forming of new border, as the African Plate slowly divides into the Nubian and Somalian Platesi. This rift began forming about twenty million years ago, and will eventually form a new oceanii.

Africa being the origin of the hominids, this has been a boon to paleontology. Geologic activity caused a series of great lakes, whose position and inflow rivers changed and moved along with the rifting, thus providing good areas for fossil deposition and preservation. The movements also mean that older deposits are now exposed, providing excellent spots for recovery. Additionally, there has also been volcanic activity, and volcanic ash is excellent both for quick burial, and for being dated radiometrically.iii

The Afar Triangle is a spot within the region, most likely a triple junction where three tectonic plates meetiv.

Olduvai Gorge , along the Great Rift Valley in Tanzania has been the site of so many important fossils and artifacts that it is known as the “Cradle of Mankind.”v The gorge is about twenty-five miles long and 350 feet deep at the mostvi, revealing roughly seven distinct beds. Its great environment for preservation is believed to be that it was at the edge of an ancient lake, where there was frequent volcanic activity and a rising and falling water level. Subsequent erosion from a stream – presumably diverted by seismic activity – has revealed these beds, which range from 2.1 million to 15,000 years ago, from pre-Homo to anatomically modern humans. They have revealed approximately four species of hominids, the earliest type of stone tools, the oldest known “man”-made dwellings, the earliest evidence for hominid consumption of mammoth, and perhaps even hominid dungvii.viii

Fossil found along the Great Rift Valley include:

“Lucy” - Australopithecus afarensis (further discussed on pg ##)

Dating

The problem of correlating fossils is compounded with the difficulties in dating them. bla bla

Recreation

Coupled with the problems and biases of finding fossils, is the difficulty of recreating individual animals, their lifestyles, entire species, and ecosystems. When a jawbone is found in Ethiopia, and a femur in Kenya, the problem of organizing finds into our knowledge and understanding becomes formidable.

Human Evolution

When trying to match up different bones found in different places into species, one is essentially working on phylogeny. With many paleontologists, anthropologists, etc. looking at the fossils, each individual find is often the subject of very differing opinions. It is through the slow process of seeking and sharing research and theories, and discovering new finds, that a consensus of information begins to build up.

The overall consensus today is that the human and chimpanzee lines diverged between five and ten million years ago (probably about six mya), in Africa. The human line divided into two main branches, which we call the genera Australopithecus and Homo. Homo erectus was the first species to spread outside of Africa, followed by Homo sapiens, which either evolved from H. erectus all over the world, or else evolved in Africa and migrated outwards as well. This last evolution took place about 100,000 years ago.

One of the problems with naming species, is that this is quite difficult with incomplete skeletons. Another is that speaking evolutionarily, when one species evolves from another, there is no clear delineation between species, but rather a gradient. For example,

“at least half of the specimens that have been assigned to Homo habilis by some scholars have been variously attributed to Australopithecus africanus or Homo erectus by others” p86

There are several big and overlapping questions regarding human evolution: when, where, and why did we develop our primary unique characteristics: bipedalism and intelligence, and in which order? We have largely solved a small part of that; it is very much the consensus that bipedalism developed prior to intelligence. Additionally, what species were our direct ancestors, as opposed to branches – did we evolve from H. erectus (and where?), and H. neanderthals?

What the hominid family tree looks like is very much disputed, and it would seem that every textbook, paper, website, etc. will show a different interpretation. The only thing that is reasonably consistent is the dates attributed to specific fossils, and so all attempts at describing the phylogeny are approximately the same chronologically. What they differ most in, is the relationships between the species.

*http://www.geocities.com/palaeoanthropology/timeline.html

*http://www.pbs.org/wgbh/evolution/humans/humankind/

*http://www.wsu.edu/gened/learn-modules/top_longfor/timeline/timeline.html

*http://www.archaeologyinfo.com/species.htm

*http://www.mnh.si.edu/anthro/humanorigins/ha/a_tree.html

Important Finds

Some of our best or most significant information comes from a number of important individual fossil discoveries. Here, some of the most well-known ones are described.

Lucy (break into two paragraphs)

“Lucy” is the name given to what is probably the most well-known individual fossil find, technically AL 288-1. It comprises forty percent of an Australopithecus afarensis skeleton. Since we can assume bilateral symmetry, a majority of the skeleton was found or can be assumed p74. Lucy was found in 1974 in the Hadar formation in Ethiopia, which falls within the Afar region. What makes Lucy special, aside from the completeness of the find, is that we can be fairly certain that she was bipedal, and was the earliest known biped until 1994. The best estimate is that Lucy dates to almost 3.18 million years ago, thanks to argon-argon (40Ar/39Ar) dating of volcanic ash. It is thus also very strong evidence that bipedalism evolved prior to a large brain. That Lucy is female is known due to her small size, as it seems that A. afarensis exhibited strong sexual dimorphism. While determining locomotion from a skeleton is a difficult task, the following diagram illustrates visually how similar A. afarensis was to H. sapiens.ix

Figure ##: Diagrams comparing the pelvis of a human and chimpanzee with that of Lucy. The chimpanzee's is long and narrow, while the other two are short and broad.x

Tuang Baby” in Botswana in 1924

Homo habilis type specimen

The fossil OH 7, found in Olduvai Gorge, Tanzania around 1962, has lead to what is perhaps the most debated hominid “species.” Louis Leakey and colleagues described their find as a new species, Homo habilis, for “handy man”, due to the supposed tool-making abilities. This is based on associated stone tools found at the site and elsewherexi. The genus Homo, arising from earlier Australopithecines (or, as some suggest, they evolved in parallelxii), is generally distinguished by the larger brain size, and H. habilis is perhaps the first of these. As previously mentioned, however, the majority of fossils attributed to H. habilis are argued to belong to A. africanus, H. erectus, or others, and the existence of this distinct species itself is questioned. Consensus today appears to be that indeed H. habilis deserves the rank of “species.” Some see it has the ancestor to H. erectus and/or H. sapiens, but this is far from clear.xiii

Laetoli footprints

The Laetoli site in Tanzania is one of our most important finds, thanks to volcanic activity. Rain upon recent volcanic ash created a sort of plaster surface, upon which many animals walked before it was quickly covered again. Among the footprints found are those of hyenas, cats, baboons, boar, giraffes, gazelles, rhinos, antelope, elephants, birds, etc., and most importantly a hominidxiv. These 3.6 million footprints – found in 1976 – are indisputably bipedal, showing an arched foot, a big toe that was not very distinct from the others, and a walking motion at least very similar to our own. They are usually attributed to Australopithecus afarensis, although this is much disputedxv. They show two individuals walking adjacent, and perhaps a third that walked in the footsteps of one of the others.xvi

xvii

“Turkana Boy” in Kenya in 1984

The Black Skull

The “black skull” (KNM-WT 17000) was found in West Turkana in Kenya in 1985, an almost complete and intact cranium 2.5 million years old, and is one of very few specimens attributed to Australopithecus aethiopicus. It was instrumental in renaming an earlier find that had been designated as Paraustralopithecus aethiopicus, and is may perhaps be somewhere between the two groups of Australopithecines, the “robust” and the “gracile.”xviii

foo

2001 discovery of a hominid cranium and associated fragments in Chad by the French paleoanthropologist Michel Brunet and his team. These fossils are dated at 6 to 7 million years old — the oldest hominid yet found. This find undoubtedly will cause a wholesale revision of evolutionary scenarios, as previously the oldest known hominid fossils were ~4.4 million years old http://www.cns.uni.edu/~groves/LabExercise13.pdf

http://www.google.com/search?q=%22most+important%22+hominid&start=0&ie=utf-8&oe=utf-8&client=firefox-a&rls=org.mozilla:en-US:official

http://www.nature.com/news/2002/020708/full/020708-11.html

http://en.wikipedia.org/wiki/Hominidae

http://anthro.palomar.edu/hominid/australo_1.htm

http://www.talkorigins.org/faqs/homs/

http://en.wikipedia.org/wiki/Human_evolution

http://www.ees1.lanl.gov/programs/gidayarch.html

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=1736671&dopt=Abstract

http://www.selamta.net/Lucy.htm

http://www.wadsworth.com/anthropology_d/special_features/ext/latest_dirt/southafrica.html

http://johnhawks.net/weblog/topics/geology/rift/rift_valley_overview.html

http://www.anthro4n6.net/lucy/