Trends in Human Evolution
Excerpts From: Human Evolution
The University of Waikato, New Zealand
Introduction:
Humans are a young species, in geological terms. The average "lifespan" of a mammal species, measured by its
duration in the fossil record, is around 10 million years. While hominids have followed a separate evolutionary
path since their divergence from the ape lineage (see classification table below), around 7 million years ago,
our own species (Homo sapiens) is much younger. Fossils classified as archaic H. sapiens appear about 400,000
years ago, and the earliest known modern humans date back only 170,000 years.
Linnaean Classification
Kingdom
Phylum
Class
Order
Family
Genus
Species
Homo Sapiens (Human)
Animalia
Chordata
Mamalia
Primates
Hominidae
Homo
sapiens
Pan troglodytes (Common Chimp)
Animalia
Chordata
Mamalia
Primates
Hominidae
Pan
troglodytes
Examination of hominid remains indicates several trends, including changes in posture, cranial capacity (brain
size), and facial angle. Such trends are often misused, e.g. in popular illustrations, to give the impression that
evolution has proceeded in a linear manner, from some primitive ancestor through a series of descendants, to
culminate in our own species. It's important to remember that the evolutionary history of humans, as of most
organisms, is best reconstructed as a bush, where there are often several related species in existence at any
one time.
Having said that, these trends do give a useful overview of the evolutionary changes that have occurred in our
biological history.
Trends in cranial capacity
Early workers in the field of human evolution expected that the first hominids would have an ape-like physique
with a modern cranium. This reflected the attitude that, since our intelligence and large brain size set us apart
from all other species, these would be the first human characteristics to evolve.
We now know that the actual trend is the reverse of this early expectation. There has been a gradual increase
in cranial capacity over the course of human evolution. Thus the early australopithecines had cranial capacities
within the range of modern chimpanzees (average around 400cc). Skulls attributed to early Homo begin at
around 510cc, and there was a marked increase with Homo
erectus, where later specimens had brain sizes of up to
1225cc, well within the modern range.
The average cranial capacity of the Neandertals was larger
than that of modern humans (1450cc and 1350cc
respectively), but this may simply reflect the larger body
mass of neanderthalensis. There is a strong positive
correlation between body size and brain size, even within species e.g. male humans have larger body mass
than females, and correspondingly larger cranial capacity. Equally importantly, the brain size in hominids,
particularly Homo species, is greater than would be predicted for animals of their body mass.
Trends in general morphology of the skull
Dental arcades: in an ape, the teeth are arranged in a rectangular dental arcade, where the left and right cheek
teeth are in two parallel lines. Australopith dental arcades tend to be more rectangular than parabolic, but in
Homo species the dental arcade is a full parabola, broader at the back than at the front.
There is also a strong trend in tooth size, such that the cheek teeth (in particular) of modern humans are
smaller than those of australopithecines. Over the last 100,000 years there has been a continuation of the
trend towards smaller molar teeth and a more gracile skeleton, such that the Upper Palaeolithic humans of
30,000 years are described as being 20-30% more robust than present-day people. This demonstrable trend in
tooth size is probably linked to the use of food-processing techniques that reduce the need for prolonged
chewing, and thus provides a good example of the results of natural selection in human populations.
Crests and ridges: both the great apes and early hominids have obvious crests and ridges on their skulls. The
most obvious are the sagittal and nuchal crests and the brow ridges.
Where present, the sagittal crests provide anchorage for large chewing muscles, and are thus most prominent
in species where the diet comprises hard, tough material requiring a lot of chewing. Brow ridge development
in hominids may also be related to diet, as large brow ridges help to redirect the considerable stresses placed
on the skull by a diet of coarse vegetable matter.
There is a general trend towards a flatter facial angle with the appearance of more recent hominids,
culminating in the vertical face of Homo sapiens. Note, though, that there is considerable variation even
among the older members of our lineage.
Bipedalism:
Bipedalism appeared very early in our evolutionary history. Until recently there was disagreement over the
posture of Sahelanthropus (c. 7 million years ago). However, a computer-assisted reconstruction of this fossil
shows a foramen magnum beneath the cranium, and relatively small nuchal crest, indicating that this species
was bipedal, although not fully erect in posture (Zollikofer et at. 2005).
In contrast, Homo erectus was possibly an even more efficient biped than modern humans, due to the
narrower pelvic outlet in erectus. The wider pelvic outlet in sapiens, an adaptation permitting the birth of
large-brained infants, places the hip joints further apart than required for optimum locomotory efficiency.
(This is a good example of how evolution produces compromise solutions, rather than perfection.)
Reduced sexual dimorphism:
All hominoids show some differences in size between the sexes, as well as in such
features as the shape of the pelvis and in crests on the skull. Thus male gorillas weigh
perhaps twice as much as females. This size difference is much less in chimpanzees
and even less pronounced in modern humans, where on average males are 1.2 times
as heavy as females.
This trend towards a lesser degree of sexual dimorphism can be traced in hominin fossils. The skeletons of the
australopithecines show a marked degree of sexual dimorphism, which is reduced in the early hominids.
Tools and Tool Use:
At present the earliest-known evidence of the manufacture and use of tools comes from a 2.5 million-year-old
site, possibly associated with Australopithecus garhi. This site contains primitive stone tools, but no hominin
remains. Animal bones recovered together with garhi remains, from a nearby site of the same age, appear to
show cut marks from stone tools.
These tools predate the better-known Oldowan tool culture associated with Homo habilis. These are often
described as "cobble tools", and comprise two main types, core tools and flake tools. Core tools are stones
with one or more flakes knocked off one end to give a jagged edge. Flake tools are the flakes removed in
producing core tools, and were not modified any further before being used. The Oldowan tool culture
persisted in Africa for almost a million years.
With Homo erectus came the more sophisticated Acheulean toolkit. These tools were more highly modified
than the earlier cobble and flake tools, with sharper and straighter edges formed by careful removal of more
and smaller flakes. Perhaps the best-known Acheulean tool is the so-called hand axe, a
teardrop-shaped implement with a pointed end and sharp sides. We have no way of
knowing just how these hand axes were used, but they were probably put to a wide
range of uses. Other Acheulean tools included hammers, cleavers, and flake-based
tools such as knives (probably held directly in the hand, rather than hafted to a
handle).
Erectus was probably the first hominin to leave Africa. Acheulean tools are found in
both Europe and Asia. Until recently it was believed that Chinese erectus populations
continued to use Oldowan tools, but Acheulean tools dating back 1.3 million years
were found in China in 2001.
Along with more sophisticated tools came a change in the foods eaten, and how these foods were obtained.
While the australopithecines, and perhaps H. habilis , were essentially vegetarian, meat was a regular part of
the erectus diet. Remains from many sites, including Zhoukoudian in China, show that erectus was eating meat
on a large scale and from a range of animal species, in addition to a wide variety of plant foods. These animals
may have been both scavenged (from other predators) and hunted.
The year-round availability of calories from meat would have made it possible for erectus to move from its
tropical homeland into temperate regions. Parts of China, for example, experience cold winters, when fresh
plant foods are not readily available and meat would be the primary source of calories.
There is also a possible causal link between the marked increase in cranial capacity of Homo erectus especially the rapid rate of growth of the brain after birth - compared to its predecessors, and the regular
presence of meat in erectus diets. The brain is a very fatty organ, and meat is a much better source of the
necessary fats than plant foods. The high calorie content of meat is also important, as the brain is a very
energy-hungry organ. (And of course, breastfeeding an infant with a rapidly growing brain is energetically very
expensive.)
Another leap in tool development came with the Mousterian tool culture, associated with both Neandertals
and archaic Homo sapiens. In a significant advance over the Acheulean culture, a stone core was carefully
shaped before flakes were struck off it: different core shapes gave different flakes. These flakes could then be
further modified for a range of different tasks, and some have a tang at the end that suggested that they were
hafted to a wooden or bone handle.
The appearance of modern Homo sapiens saw further innovation with a new group of tool-making styles,
collectively known as the Upper Palaeolithic industry. The earliest such tools date from Africa, around 90,000
years ago.
Other cultural artifacts are associated with the later Gravettian, Solutrean, and Magdalenian tool industries.
Ivory beads and "Venus" figurines are associated with Gravettian sites, while necklaces, animal figurines, and
symbolic art appeared during the Magdalenian, 18,000 to 12,000 years ago.
Language & Culture:
This hard evidence of human cultural evolution can be used to infer something about the nature of the human
societies producing these artifacts. Needles suggest the manufacture of relatively sophisticated clothing, as
does the use of huge quantities of beads to decorate these clothes (found in some grave sites).
Palaeoanthropologists have linked the nature of many cave paintings and carvings to the development of
various rituals - and also to the development of fluent abstract language. They argue that the complex and
often abstract nature of much cave art could not have been developed without an equal ability to
communicate. Similarly, the appearance of complex burials at some Upper Palaeolithic sites may imply
concerns or beliefs about an afterlife.
We think of complex culture as a hallmark of humanity. However, art works, such as jewellery, carving, and
cave paintings do not appear in the record until 30-40,000 years ago. This follows the development of the
extremely sophisticated Aurignacian tool kits associated with Cro-Magnon culture. Some scientists suggest
that the use of highly sophisticated language accompanied this flowering of culture. This is not to say that
earlier humans, and hominids, were not capable of speech.
A Selection of Hominids:
Australpithecus afarensis: The best-known member of this species is "Lucy" , discovered in 1974 by
Don Johanson & Tom Gray and estimated to be around 3.2 million years old (afarensis lived from
3.9 to 3 million years ago). She is named for a song playing on the radio during the celebration of
her discovery: Lucy in the Sky with Diamonds. Afarensis remains indicate that the species was
strongly sexually dimorphic, which may give us some clues about social behaviour in afarensis, since
modern apes with a high degree of sexual dimorphism are polygynous. The face and cranium of
afarensis was ape-like; the teeth are intermediate between ape and human.
Australopithecus garhi: Its discoverers felt could be intermediate between A. afarensis & early Homo, was
described in 1999 from fossils found in the Awash region of Ethiopia. The 2.5 million-year-old fossils were
found in association with animal bones - which had marks on them that appeared to be from stone tools. Very
simple stone tools were found in a nearby site of the same age.
Homo habilis: lived from 2.4 until 1.5 million years ago, and closely resembles the
australopithecines. In fact, recent papers have suggested that habilis would be more
appropriately classified in Australopithecus. The face still projects forwards but the facial angle is
less than in A. africanus. Average cranial capacity in habilis is about 650cc, and the range is 500 800cc - this overlaps both the australopithecines (at the lower end) and H. erectus (at the upper
limit). Analysis of wear patterns on the teeth suggest that habilis was adding meat to its diet probably as a scavenger as there is no evidence that hunting was a common practice.
Homo erectus: lived between 1.8 million and 300,000 years ago, and was probably the first hominid species to
move out of Africa and colonize Europe and Asia. This significant event must have happened early in the
species' history, as fossils that may belong to erectus but which are dated at around 1.8 million years ago have
been found in Dmanisi, Georgia. Erectus had a long, low skull, with little forehead
and a cranial capacity of between 750 and 1225cc. The smaller brain sizes are
associated with older specimens. The face had a projecting lower jaw that
supported large molar teeth but lacked a chin. This was the first hominid to have a
projecting, rather than a flattened, nose. The post-cranial skeleton of Homo
erectus was robust, suggesting they were stronger than modern humans.
Members of this species, at least in Africa, were tall; some estimates place them in
the upper quartile of the height range for H. sapiens. However, the few remains
from China ("Peking man") are from shorter individuals. This probably reflects
adaptation to the local climate. Tall, slender individuals are well adapted to lose heat in hot climates, as their
bodies have a high surface area to volume ratio (SA:V). Those living in colder regions need to conserve heat,
and heat loss is reduced in short, stocky individuals with a lower SA:V.
Homo neanderthalensis: Neanderthals lived between 230,000 and 30,000 years ago, during
the last Ice Age, and were found only in Europe and the Middle East, where they coexisted
with modern humans for the later part of their existence. This species gets its name from the
Neander valley, or Tal, in Germany, where the type specimen was found in 1856. All
Neanderthals are heavily built but those from Western Europe are particularly robust. Their
heavy physique was probably an adaptation to the extremely cold conditions in which they
lived. Recent genetic studies support the hypothesis that Neanderthals and early humans
interbred.
Homo sapiens (modern): A recent find provides good evidence that the earliest known recognizably modern
humans lived in Africa, around 160,000 years ago. These fossils come from Herto, in the Middle Awash of
Ethiopia (White et al. 2003). The fact that the Herto fossils are so old supports the argument that fully modern
humans first arose in Africa, later migrating into Europe & Asia to displace the other hominid species already
living there (the "out of Africa" hypothesis). The average cranial capacity of modern humans is around 1350cc,
with a range for normal individuals of from 800 to close to 2000cc. The brain is enclosed in a high, vaulted skull
with a high forehead, and brow ridges are absent or, if present, very small. The relatively delicate jaw has small
teeth and a prominent chin, and the post-cranial skeleton is gracile.
Excerpts From: Human Evolution: http://sci.waikato.ac.nz/evolution/HumanEvolution.shtml#Trendsinhumanevolution
The University of Waikato, New Zealand; Lexile 1400
CIS Unit
Trends in Human Evolution
(9th and 10th grade Biology)
SC.912.L.15.10: Identify basic trends in hominid evolution from early
ancestors six million years ago to modern humans, including brain size,
jaw size, language, and manufacturing of tools.
Essential Questions:
o What are some basic trends in hominid evolution?
o What is the taxonomic classification of man?
CIS Lesson
Trends in Human Evolution
1. Hook Engage: Finding Lucy http://www.pbs.org/wgbh/evolution/library/07/1/l_071_01.html
2. Question #1
Predict whether human evolution is best reconstructed in a linear manner (e.g., from some
primitive ancestor through a series of descendants, to culminate in our own species) or if it is
best reconstructed as a bush, where there are often several related species in existence at any
one time.
3. Distribute article.
4.
a.
b.
c.
d.
e.
Pre-teach vocabulary:
Australopithecine
Cranial/cranium
Culminate
Dental arcade
Foramen magnum
f.
g.
h.
i.
j.
gracile
Hominid
Hominin
Hominoids
Morphology
k. Neanderthal
l. Nuchal crest
m. Sagittal crest
5. Text-marking:
A = Appearance
B = Behavior
6. Question #2
Construct at least one paragraph explaining why human evolution is best reconstructed as a
bush and not linearly as previously believed. In your response, be sure to provide examples of
overlapping species and characteristics they may or may not share with one another.
7. Note-taking: Guiding question “What are the major trends in human evolution?” Identify the
species for which they are associated and the relative time period for their appearance or
disappearance.
8. Task #3
You have been commissioned to create a product that details the trends in human evolution.
You may create a graphical timeline, digital storyboard, PowerPoint, or webpage. For each
species, along with a picture, construct at least one associated paragraph that details the
physical and behavioral characteristics of that species along with the time period in which that
species lived. For each species, provide information on the major trends as they existed in that
species and how it evolved from earlier species. The major trends to note are: changes in the
morphology of cranium and skull, bipedality, sexual dimorphism, tool use, and language &
culture.
#1 Predict whether human evolution is best reconstructed in a linear manner (e.g., from
some primitive ancestor through a series of descendants, to culminate in our own species) or
if it is best reconstructed as a bush, where there are often several related species in
existence at any one time.
#2 Construct at least one paragraph explaining why human evolution is best reconstructed
as a bush and not linearly as previously believed. In your response, be sure to provide
examples of overlapping species and characteristics they may or may not share with one
another.
Directed Note-Taking
Trends in Human Evolution
Directions: Record notes containing the most important information relevant to the guiding question.
Guiding Question: “What are the major trends in human evolution?”
Identify the species for which they are associated and the relative time periods for their appearance or
disappearance.
Check Relevant Categories
Page/
Paragraph#
Notes
Species
Trend in
Evolution
Collaborative Work: After completing your chart, be prepared to compare your notes with others.
Time
Period
Additional Notes Page
Page/
Paragraph#
Notes
Species
Trend in
Evolution
Time
Period
Question Generator
Directions: Carefully review the text for words, phrases or statements that create questions in your mind.
Discuss these questions in your group, and then document your group’s questions below.
Trends in Human Evolution
Do your questions pertain to any of the categories on the right? If yes, place a check in the appropriate box.
Page/
Paragraph#
Notes
Collaborative Work:
After completing your chart, be prepared to compare your notes with others.
Species
Trend in
Evolution
Time
Period
Directions: Write your response to the following question using information you learned in this unit. You may also use
research from additional outside sources. Be sure to cite those sources!
#3 You have been commissioned to create a product that details the trends in human evolution. You may
create a graphical timeline, digital storyboard, PowerPoint, or webpage. For each species, along with a
picture, construct at least one associated paragraph that details the physical and behavioral
characteristics of that species along with the time period in which that species lived. For each species,
provide information on the major trends as they existed in that species and how it evolved from earlier
species. The major trends to note are: changes in the morphology of cranium and skull, bipedality, sexual
dimorphism, tool use, and language & culture. Use the space below to begin sketching out your ideas.