Human Evolution
Assessment Statement
D.3.1 Outline the method for dating rocks and fossils using radioisotopes, with reference to 14C and 40K
D.3.2 Define half-life
D.3.3 Deduce the approximate age of materials based on a simple decay curve for a radioisotope
D.3.4 Describe the major anatomical features that define humans as primates
D.3.5 Outline the trends illustrated by the fossils of Ardipithecus ramidus, Australopithecus, including A. afarensis, and A. africanus, and Homo, including H. erectus, H. neanderthalensis and H. sapiens
D.3.6 State that, at various stages in hominid evolution, several species may have coexisted
D.3.7 Discuss the incompleteness of the fossil record ant the resulting uncertainties about human evolution
D.3.8 Discuss the correlation between the change in diet and increase in brain size during hominid evolution
D.3.9 Distinguish between genetic and cultural evolution
D.3.10 Discuss the relative importance of genetic and cultural evolution in the recent evolution of humans
Humans are known as Homo sapiens (modern man). The full classification is:
Kingdom: Animalia
Phylum: Chordata
Subphylum: Vertebrata
Class: Mammalia
Subclass: Eutheria
Order: Primates
Suborder: Anthropoids
Family: Hominidae
Genus: Homo
Species: Sapiens
The fossil record, allows us to look at common morphology and deduce common ancestry. The relatedness of organisms is investigated by comparative biochemical studies, particularly of mitochondrial DNA, which in each generation is passed from mother to offspring unchanged. This type of DNA undergoes a steady rate of mutation – it changes as a function of time alone. The degree of difference between mitochondrial DNA samples discloses how recently groups of organisms shared a common ancestor.
Dating Rocks and Fossils
Fossils can tell us a lot about the past.
Fossil – any form of preserved remains from a living organism.
Some examples are:
· Mammoths frozen in Siberia
· Mummies in acidic swamps in Scandinavia
· Insects in amber
· Bones in rock
Fossils are only formed in some circumstances. Most individuals do not leave a fossil after death.
A fossil has to be formed when an organism dies and gets buried in sedimentary silt. It will decay slowly and leave a space in the silt. The gap becomes solid and is filled the exactly the same as the organism left behind. The silt may solidify, becoming sedimentary rock and in it is the fossil.
To see how old fossils are and their forms, carbon dating is used, usually Carbon 14 and potassium 40, which are isotopes.
Isotopes are atoms of the element that have different numbers of neutrons. Therefore, they are unstable and will spontaneously change into one or more atoms to other elements, often emitting some radiation. The time taken for this change is determined by the kind of isotope. After a period of time, at a fixed interval, the radioactive decay will be half of what it was before. This is called half-life. For C14, the half-life is 5730 years.
Using C14
Most carbon is C12, but due to cosmic radiation C14 is formed at a low, steady rate. While alive, organisms absorb carbon in the ratio of C12 / C14 present in the environment around them. After death, accumulation of radioactive (and other) atoms stops. Meanwhile, C14 steadily breaks down:
half-life of 5.6 X 103 years
C14 N14
So the ratio of C14: C12 in a fossil decreases with age; the less C14, the older the fossil.
This technique gives good dates for fossils of the last
60 000 years.
Using the ratio of K40:Ar40
Rocks do not eat or photosynthesize. Some contain no carbon at all. Instead we use potassium – 40. The pyroclastic rocks flowing out of volcanoes may contain radioactive isotopes such as potassium-40, which decays to argon-40, as shown:
half-life of 1.3 X 109 yrs
K40 Ar40 (gas)
In hot lava, argon gas boils away into the atmosphere. Once lava has solidified by cooling, which occurs quickly after volcanic eruptions, the argon gas that is then formed by radioactive decay is trapped in the rock.
By measuring the ratio of K40: Ar40 in lava deposits, the exact ages of the lava and the approximate age of the sedimentary rocks (and their fossils) below and above lava layers are estimated.
This technique spans the whole of geological time back to the Cambrian period (580 million years ago), but it is too slow to give reliable results over the most recent half million years.
Determining the age of a fossil or rock.
You look at the percentage of Carbon – 14 or potassium – 40 left in the fossil or rock. If there is 50% left, that means there has been 5730 years past for C14. For 25%, that would be double the half-life or 11 460 years. Another way, is to look at a decay curve and see where the amount of remaining C14 falls on the curve to estimate the time.
Humans as Primates
Humans belong to the mammalian order Primates. This order contains three distinctive groups of animals, namely the apes (which includes the genus Homo), the monkeys, and the prosimians (a name meaning ‘before the monkeys’). These are mostly tree-dwelling species with grasping hands and feet. The range of animals that constitute the Primates and how they are related are summarized below.
Apart from humans, who have achieved worldwide distribution, most primates live in tropical and sub-tropical regions. An interesting feature of primates is their relatively unspecialized body structure, combined with some highly sophisticated behavior patterns.
Why are humans defined as primates?
To the biologist, humans are primate mammals. By this we mean that humans show many of the characteristics of other mammals, the general characteristics common to other primates, and many of the features shown by the great apes to which we are most closely related.
Major features, which describe humans, as primates are adaptations to tree life. They are the opposable thumb, acute vision (stereoscopic vision), mobile arms and shoulder girdle and a skull modified for upright posture.
Having an opposable thumb means you can manipulate objects and be able to grasp. Mobile arms, allows movement in three planes and transfer weight via the arms. This is very important for tree dwellers and for movements above the head. Also, living in trees means you can see further. As a result, the eyes are places more forward, on a flat front. This gives a smaller field of vision, but more acute and the overlap of vision allows for good depth perception and judgment of distance. Along with this is color vision. The positioning of the magnum foramen (mentioned later in these notes), allows for the spinal column to insert into the skull in a more upright position, lessening the curvature of the spine and allowing primates to walk upright.
Origins of Humans – Trends in hominid and human fossils
The earliest fossils which we confidently identify as anthropoids (apes) have been found at many sites in Africa. They date from about 35 million years ago (mya). Humans clearly demonstrate one form of anthropoid body organization, so we can say the human story has taken about 35 million years to unfold.
There are some similarities and some major differences between all the hominid skulls that have been unearthed. The details to pay close attention to are summarized below. One key area is the foramen magnum. It is the hole where the skull is attached to the spinal column. In modern humans the hole is in the center of the base of the skull, giving rise to the theory of walking upright. In apes, the hole is further back to accommodate the spinal column in an animal that walks on four legs.
There is some discussion as to what a hominid is. It can only refer to those bipedal primates, which are direct ancestors of modern humans.
The chronological order of some of the species of hominid for which have been found is below.
Fossils of Hominidae have lead to several speculations about evolution.
1. Ardipithicus ramidus
a. Lived approximately 5.8 – 4.4 mya in Ethiopia. This species is believed to be very close to the split between the line of organisms, which became more human-like and the line, which became more chimpanzee-like. Most of the fossils are teeth and therefore, it is difficult to be sure. From what has been found, the Ardipithicus ramidus was very similar to a chimpanzee with a few hominid features. The molars show more ape-like characteristics, as the length is greater than the breadth. The canines are more hominid, as they are shorter and not as sharp as ape canines.
2. Australopithecines (southern ape) lived about 4 mya. They had 500 cm3 brains and walked upright.
a. The first species was A. afarensis from the Afar desert (4-2.8 mya) found in Ethiopia and Tanzania. (Lucy skeleton) It had a tall lower jaw, fairly large molar teeth and a projecting face. The cranial capacity was 380 – 430 cm3.
b. Later came africanus (3-2 mya) found in South Africa. It is thought to be the same species as afarensis as features are similar and walked upright. It had a tall, thick lower jaw, large molars and a projecting face. The cranial capacity was 435 – 530 cm3.
c. Later was A. robustus (2-1.4 mya) in South Africa. They were larger and heavily built.
3. Then came the Homo genus. They were from around 2 mya and had larger brains (600 cm3) and walked upright.
a. First was H. habilis (handy man). It is thought he arose from A. afarensis 2 mya in East Africa and used simple tools. It had a flatter face, larger molars but the cranial capacity was still only about 600 cm3.
b. Homo erectus was from Africa. It is thought it migrated to other parts of the world and had a larger brain than H. habilis. H. erectus spread to Asia and Europe. However, it is believed that H. sapiens evolved at one place in Africa and from their spread out over the world. It has a smaller jaw, a receding forehead, large brow ridges and smaller molars. Its cranial capacity was 1000 cm3.
c. H. neanderthalensis, which lived in Eurasia from 200 000 to 30 000 years ago. The species survived several ice ages. It has a smaller jaw, a lower forehead, smaller brow ridges and smaller molars than the previous species. They had larger brains than modern humans, with a cranial capacity of up to 1600 cm3.
d. Next was H. sapiens, which came to Europe. The first subspecies was Cro-Magnon man, who looked a lot like modern humans and though to have used the first language. H. sapiens lived around 140 000 to 70 000 years ago in Africa and Asia as well. They had a high forehead, no brow ridges, a flat face, small molars and a very small jaw. This species developed cave paintings, tools and weapons. The cranial capacity was similar to today’s humans of 1300 cm3.
Based upon where the skulls were found and dating, we can see that many species may have coexisted. As the Homo sapiens developed, so were the homo neaderthalenis.
The incompleteness of the Fossil Record
Anthropologists disagree about the origin of modern humans from time to time. They use evidence from fossil remains, from artifacts like stone tools that can be associated with particular hominids, and the record in animal bones that surrounded their habitations and which indicate diet. Fresh evidence of these types is frequently discovered, and existing data are sometimes reinterpreted.
Re-interpretation occurs in the light of new biochemical evidence or the development of new analytical techniques. For example, until quite recently, another theory about the origin of modern humans vied with the current ‘out of Africa’ theory.
The alternative was a multiregional model, in which H. sapiens emerged wherever populations of H. erectus had become established, in Africa, Europe and Asia. This made H. neanderthalensis only one example of an archaic hominid form, intermediate between H. erectus and modern humans. According to this model, there was ongoing genetic exchange between populations of various archaic forms until H. sapiens emerged and replaced all others. Currently, the body of evidence is increasingly against this theory.
Controversy will continue because of the inevitable incompleteness of the fossil record. Fossilization is an extremely rare, chance event. This is because predators, scavengers and bacterial action normally break down dead plant and animal structures long before they can be fossilized. Of the relatively few fossils formed, most remain buried, or if they do become exposed, are often overlooked or may be accidentally destroyed before discovery.
Nevertheless, numerous fossils have been found, and as more hominid fossils are discovered, so our knowledge may change and our understanding of our past be advanced. This is yet one more branch of science where the frontier of knowledge is entirely open. You can follow the debate from now on.
Brain Size
So why did the brain develop? Some think because the environment was so diverse, a larger brain was needed to deal with the challenges, and therefore, larger brains were selected as an advantage.
Habilines were the first hominids to be associated with tools – they used large pebbles, chipped in at least two directions, as sharpened implements to crush, break and cut. Their additional brain capacity had resulted in advanced manual dexterity. It was applied to the making and using of simple tools (selected strong stones) to chip pebbles, for a purpose. Using tools to make tools (i.e. the development of a tool industry) is what distinguishes hominid toolmakers from all other tool-users in the living world.
Skull endocasts (casts of the inside of the brain case of the skull) show that the areas of the brain associated with speech and language are significantly developed, so we can assume that cultural evolution was also under way. This was also the first hominid to use fire consistently, which will have aided the colonization of areas so far north of equatorial Africa, and also with its habit of eating meat.