The origin of the hominins takes place in Africa



Download 113.61 Kb.
Date conversion27.05.2016
Size113.61 Kb.


Chapter 10

The First Hominins


Overview

  • The origin of the hominins takes place in Africa.

  • All of these early hominins had small brains but were bipedal.

  • The origin of bipedalism is a major event in hominin evolution for which a number of hypotheses have been proposed.

  • This chapter examines the beginning of the story of human evolution focusing on the earliest hominins prior to the appearance of the genus Homo.

  • General overview of hominin events:

  • Answering the question ‘how old are human beings?’ can be answered in different ways. Part of the diversity in answers is based on what we are calling human

  • Roughly 200,000 years for those more or less anatomically the same as living human beings

  • All large-brained humans, several hundred thousand years; all members of the genus Homo, more than 2 million years

  • All bipedal hominin, 6+ million years.

  • Humans emerged over time as a byproduct of many different biological and cultural evolutionary changes in a mosaic fashion.


Overview of Human Evolution 1

  • The study of human evolution is both interesting and confusing.

  • New names, new places abound.

  • I urge you to create a set of tables into which to type the names and dates from the next several chapters.

  • You might also go here: http://highered.mheducation.com/sites/0078034981/student_view0/index.html

  • Arguments between taxonomists flourish.

  • At left is a broad story of human evolution. Note the sequence of new traits/cultural inventions.


Overview of Human Evolution 2

  • A timeline of human evolution

  • The hominin line split from the African apes about 6-7 million years ago.

  • There are several possible candidates from around 6 mya.

  • They were bipedal and walked on the ground, but also climbed in trees.

  • Over the next several million years one or more primitive hominins lived in Africa, most of which were of the genus Australopithecus

  • By 3 million years ago, two distinct lines of hominin had evolved.

  • One line led to several species known as “robusts” which were adapted to a diet of foods hard to chew. Extinct by 1.4 mya

  • The other line began to rely more and more on learned behavior and by 2-2.5 mya had evolved into the first members of the genus Homo appeared sometime between 2.5 and 2.0 million years ago.

  • Homo erectus was the first hominin to expand out of Africa by 2 million years ago

  • It was the first hominin to expand out of Africa, hunted, used fire and introduced a new tool tradition (Acheulian).

  • Essentially modern skeleton, fully bipedal, and a brain size about 70% of modern.

  • Homo heidelbergensis appeared around 800 kya. Found in parts of Africa, Asia and Europe it had a large brain, hunted and created new tools by 300 kya.

  • In Europe and Middle East we see the introduction of Neanderthals.

  • In Africa by 200 kya modern humans emerged, and by 100 kya migrated out of Africa, Australia by 60 kya, and the New World by 15-20 kya


Box 10.1: The Piltdown Hoax

  • Remember that not all modern traits appeared at the same time

  • We know bipedalism began 3.5 my before an increase in brain size or use of tools.

  • At one time the reverse was thought to be true!

  • Brains first hypothesis: The idea that the brain developed and then prompted the development of bipedalism and tool use.

  • Piltdown fossil was ‘discovered’ at Piltdown, England between 1911-1915

  • Along with the skull was a grouping of stone tools

  • Large skull and apelike jaw that supported the Brains first view

  • The brain-first hypothesis argued that the brain would have enlarged before bipedalism appeared

  • We now know, of course that bipedalism came first.

  • Even though A. africanus verified the presence of bipedalism, it was contested for decades (in part because of Piltdown man)

  • In 1953 fluorine dating showed Piltdown to be a hoax and a human skull with orang mandible

  • Who did it?

  • Some suggest Martin Hinton, curator at the Natural History Museum in London

  • Some even suggested the author of Sherlock Holmes books (Sir Arthur Conan Doyle) as he hated archaeologists


Bipedal Adaptations


The Origin of Bipedalism 1

  • I urge you to review the short discussion in Chapter 7 on bipedalism as you review this material.

  • Bipedalism is the oldest trait unique to humans.

  • It dates to at least 4.2 mya, but there is strong evidence for a date closer to at least 6 mya.

  • The expansion of the brain did not begin until 2-2.5 mya.

  • So brain-first hypothesis is out.

  • While other primates do walk bipedally, we do it all the time.

  • Even so, early hominins did retain some ability to climb and so were not obligate bipeds.

  • Locomotion in the common ancestor

  • How did the ancestor of the earliest hominins move?

  • One idea is that common ancestors of hominins and apes was a generalized climber (similar to the orangutan).

  • They would be capable of suspensory climbing and hanging. They could stand up to reach fruits.

  • This would require apes to become knuckle-walkers and hominins to become bipeds (two shifts)

  • The second idea is that they were knuckle-walkers.

  • This is the idea that the apes retained the knuckle-walking adaptation and that hominins shifted.

  • This interpretation is more parsimonious, but may not be accurate.

  • The environmental context

  • Original idea was called the savannah hypothesis. That bipedalism was a response to the introduction of new environment

  • Today, the earliest hominins preceded the savannah and lived in a mixture of forests and woodlands

  • During the time of the first bipeds, there was climate change and so environmental change occurring.

  • The environment was more patchy. So they needed to travel between patches of forest/woodlands.


The Origin of Bipedalism 2

  • Why bipedalism?

  • Darwin suggested bipedalism was linked to tool use.

  • Bipedal locomotion freed the hands for carrying objects and for making and using tools

  • Tool use model is rejected as bipedalism was clearly developed well before tool use.

  • His idea suggested it increased infant numbers

  • More controversial were his ideas that monogamous family groups, with males hunting, females nurturing.

  • Predator avoidance was a third suggestion, but this was linked to savannah so out of date

  • Same problem for heat stress: linked to savannah.

  • Energy efficiency is another hypothesis

  • Bipedal walking is an efficient means of covering long distances, and when large game hunting came into play, further refinements in the locomotor complex may have been favored.

  • More efficient at 2.9 km (chimp walking speed) and 4.5 km (human walking speed)

  • One estimate is that early hominin would save 50% of energy as compared to a knuckle-walker.

  • Costs: Force of gravity is concentrated on very small areas, creating intense pressures on our vertebrae and feet. This means back problems, fallen arches, and hernias. Bipedalism does not allow for very fast running

  • Benefits: Energy efficient, flexible, maneuverable.


The Origin of Bipedalism 3

  • Why bipedalism? (continued)

  • Food acquisition model is based on observations of chimpanzees

  • They tend to stand upright when foraging for fruits on lower branches

  • So it may have developed as a feeding behavior and expanded to a way of moving on the ground in forests and woodlands.

  • Others suggest that bipedalism developed in the trees, based on the orangutans.

  • They often walk on branches for balance

  • Same for reaching for fruits.

  • Review:

  • Likely that there is no single cause for bipedalism.

  • Remember all primates stand upright at times.

  • It is the concept of obligate or habitual bipedalism that is the focus of the questions about human origin of bipedalism.


The Earliest Hominins 1

  • This chapter covers the time frame from 6+ to 1.5 mya.

  • This includes parts of the Miocene epoch, the Pliocene epoch (5.3-2.6 mya and the part of the Pleistocene epoch (2.6-0.01 mya).

  • All of the earliest of the hominin species have been found at sites in Africa, validating Darwin’s suggestion that this continent was the birthplace of humanity.

  • Almost all of the finds are in east and south Africa, but one is from Chad (central Africa).

  • Once called the family tree, what with all the new species found since the 1990s we now use the term evolutionary bush.

  • The number and locations of recent fossil finds is staggering. Just as much to your instructor as to you!

  • The questions that remain unanswered are numerous, but there is one of paramount importance:

  • Are these many fossil finds one genus or several genera?

  • I don’t know, but be sure that what I teach you today is gone tomorrow.

  • There are some general characteristics of all early hominins we should keep in mind:

  • Bipedalism

  • Small, ape-like cranial capacities

  • Body weights generally less than 100 pounds

  • Average height between 3’5” to 5”


The Earliest Hominins 2

  • Sahelanthropus tchadensis

  • Central Africa (Toros-Menalla) In 2001, in northern Chad, they found Sahelanthropus tchadensis

  • Probably closer to 7 mya than 6 mya, but needs more confirmation.

  • No post-cranial bones have been found.

  • Fragmentary fossil jaw and teeth resemble an ape.

  • The small canine is a hominin trait

  • Lacks the ape trait of the honing complex (shearing canine/premolar arrangement)

  • Ape-sized brain and non-protruding face and canine more hominin.

  • The braincase is small (320-380 cm3) and upper canine is reduced (derived)

  • It is massively built, with huge brow ridges, a crest on top and large muscle attachments.

  • Suggestion of bipedalism from the base of the skull.

  • The foramen magnum is the hole in the skull where spinal cord enters brain

  • While debated in Sahelanthropus tchadensis, many suggest a more vertical placement of the skull (means bipedalism)

  • Orrorin tugenensis

  • East Africa is where are 2 areas of east Africa that contain significant early hominin fossils

  • The first is in the Tugen Hills area of central Ethiopia

  • The second is the Middle Awash area of northeastern Ethiopia

  • Orrorin tugenensis was found in Tugen Hills of Kenya (East Africa), lived between 7-6 mya.

  • Fossils include some dental remains

  • Fragmentary leg and arm bones found

  • The leg bone has groove evidence for muscle attachment. Indicates bipedalism.


The Earliest Hominins 3

  • Ardipithecus (genus name means ground ape) was found in the Middle Awash, Ethiopia, lived between 5.8 and 4.4 million years ago.

  • Two species have been identified:

  • Ardipithecus kadabba (species name means basal family ancestor) dates back to 5.8-5.2 mya

  • Few fossils, mostly teeth and a few other skeletal fragments.

  • Differs from Ardipithecus ramidus as it has more primitive canines.

  • Ardipithecus ramidus (species name means root) dates to 4.4 mya species

  • If ever there is an example of how fast things change in paleoanthropology this is the one

  • The previous edition of your book stated the finds are mainly dental and cranial remains, small ape-like brain, and indication of bipedalism.

  • Major announcement in 2009: There was a reconstruction of A. ramidus that took 15 years. Skeleton found is surprisingly complete; found close to where “Lucy” lived.

  • New data is now available and it adds significantly to the fossil record (Discovering Ardi).

  • It is a female, had an estimated cranial capacity of 300-350 cm3, was about 4 feet and weighed 110 pounds.

  • The brain was small and ape-like with a cranial capacity of about 300-350 cm3

  • The midface protrudes like an ape, the lower face does not.

  • The molar teeth are smaller than in apes, but have the thin enamel of apes

  • The canines are less ape-like in shape and size

  • Ardi appears to have been both bipedal and a climber (arm length are about the same as leg length)

  • Big toe is divergent, unlike in humans, suggesting they climbed on the top of branches.

  • Probably did NOT suspend from the branches as do apes.


Australopithecus 1

  • Important question: What happens after Ardipithecus?

  • There were likely 3 genera of hominins dating between 4.2-1.4 mya: Australopithecus, Homo, and Paranthropus.

  • The next three chapters deepen the discussion of the Homo line, the genus that is characterized by an enlarged brain.

  • The remainder of this chapter discusses the other two genera.

  • Australopithecus (means southern ape) -- 4.2-1.8 mya

  • Anatomist Raymond Dart discovered an early hominin form in 1924.

  • It had humanlike teeth and walked upright, but an ape-sized brain.

  • Dart named it Australopithecus africanus.

  • Characteristics of Australopithecus

  • They had small brains (larger than Ardi, but smaller than humans); cranial capacity ranged from 400-450 cm3

  • They are called primitive due to apelike teeth (large back teeth compared to front teeth); over time the canines became more human-like.

  • Smaller than modern humans. Using Australopithecus afarensis and Australopithecus africanus estimates are that:

  • Adult males weighed 95 pounds and stood 4’9”

  • Adult females weighed 65 pounds and stood 3’7”http://www.creationism.org/books/taylorinmindsmen/taylorimmicdentalmanvsapem.jpg

  • They were bipedal


Australopithecus 2

  • Australopithecus anamensis (from anam (lake) in Turkana language)

  • Found in Kenya and Ethiopia (South Africa), dating back 4.2 to 3.9 million years ago.

  • First found on the shores of Lake Turkana, Ethiopia.

  • Most of the evidence is dental, but also some arm and leg boneshttp://www.creationism.org/books/taylorinmindsmen/taylorimmicdentalmanvsapem.jpg

  • A few postcranial pieces of fossils were found and they verify bipedal locomotion, especially the tibia (See Figure 10.11 for an illustration of this).

  • Dental traits more primitive, apelike including a large canine and a sectorial lower first premolar.

  • The back teeth are in parallel rows; the back teeth in human are more parabolic.

  • FYI: Here is a quizlet vocab list that may be of use

Dental arches


  • An excellent transition between Ardipithecus and later hominin species, given its age and mixture of primitive and derived traits.

  • In particular, the teeth become more human-like over time.


Australopithecus 3

  • Australopithecus afarensis (from Afar Triangle, Ethiopia) -- 3.7 to 3.0 mya

  • Locations:

  • Found by Donald Johanson in the 1970s. First found at found in Hadar, Ethiopia (East Africa),

  • Slightly in time, at Hadar (Ethiopia) and Laetoli (Tanzania) we find more complete specimens.

  • Most of these have been known since the mid-1970s. Several hundred specimens, representing a minimum of 60 individuals have been excavated at Laetoli and Hadar.

  • The Laetoli footprints

  • (discovered in 1978, dating to 3.7-3.5 mya) found in volcanic ash (A. afarensis); literally thousands of prints found there of many species

  • A 75-foot trail of hominin footprints (2-3 persons)

  • The impressions verify bipedalism, but of a different kind. Their stride, cadence and speed of walking were different.

  • Debate over how bipedal.

  • Apelike tendencies such as long arms, curved finger and toe bones

  • The shoulder anatomy is also more apelike

  • Two ideas:

  • They may simply be retained, but a full obligate biped.

  • They may suggest a difference in how bipedal and some time in the trees. Teeth less primitive than A. anamensis but still more primitive than genus Homo.


Australopithecus 4

  • Australopithecus afarensis (continued)

  • Cranial capacity is between 400-500; skull has the look of a small ape; but more apelike in the skull (small brain and jutting-out face).

  • The dentition is less primitive than in earlier hominins

  • Canines intermediate.

  • Upper jaw has a diastema between the incisor and the canine, The lower premolar is pointed and has 1 cusp

  • Serves to sharpen the upper canine.

  • Is intermediate to the 2-cusp (bicuspid) of modern line

  • The most famous of the A. afarensis specimens is Lucy.

  • Named after the Beatles song, Lucy in the sky with diamonds (LSD)

  • Lucy was quite short (3.5 feet) female with what some call significant sexual dimorphism.

  • She was 40% complete, including many key fragments that gave clues to bipedal locomotion.

  • At the time of her discovery (along with the Laetoli footprints) the date of first bipedalism was moved back by at least 1 million years or more.

  • New find announced in 2006 of an infant skeleton

  • This is the first infant skeleton found to be older than 100,000 years. (dates to 3-3-3.2 mya)

  • Found at Dikaka in NE Ethiopia and nicknamed Selam is estimated age as 3 years

  • “Mixed” pattern of locomotion; skeleton is very similar to that of an adult .

  • A second new find dates to 3.6 mya was found at Woranso-Mille, north of Hadar.

  • Represents the partial skeleton of a male.

  • Confirms that sexual dimorphism was present and that the locomotion pattern was mixture of terrestrial and ability to move in trees (but not strictly arboreal form of locomotion). Think transitional form.


Box 10.2: The Flat-faced Man from Kenya

  • Kenyanthropus platyops from Lake Turkana in Kenya (South Africa) dating back 3.5 to 3.2 million years ago.

  • Maeve Leakey found in 2001

  • Name translates as ‘the flat-faced man from Kenya’

  • Shows a mixture of primitive and derived features.

  • Primitive traits: small brain, jutting lower face, and small ear hole.

  • Derived traits: small molar teeth, flat face, and tall cheek region.

  • Controversial designation challenged by fragmentary and distorted skull.


Later Australopithecus Species/Early Homo 1

  • Between 3-2.5 mya an adaptive radiation of early hominins occurred.

  • The apelike teeth disappeared, but still larger than moderns

  • Some evolved immense back teeth.

  • One of the lines evolved into Paranthropus, which lived in Africa between 2.5 and 1.4 million years ago.

  • The other line lead to more species of Australopithecus and later evolved into the Homo line.

  • In Australopithecus, by 3 mya, the ape-like teeth were gone, but the size tended to be larger than in modern humans.

  • Later Australopithecus was bipedal, but still had small brains.

  • There is indirect evidence of tool use. Box 10.3: Did Australopithecus make and use stone tools?

  • Examples of stone tools themselves date to 2.5 mya in Africa.

  • There had not been much support for Australopithecus using tools, but there had been a debate for decades as to who was using the 2.5 mya tools.

  • Update 1: Date of first tool use is pushed back to 3.39 mya by the find of bones with cut marks at Dikaka, Ethiopia. Most likely use was one of the australopiths.

  • Update 2: They found the Dikaka tools (May 2015).


Later Australopithecus Species/Early Homo 2

  • Three species have been suggested as ancestral to the genus Homo: Australopithecus africanus, Australopithecus garhi and Australopithecus sediba.

  • Australopithecus africanus (southern ape of Africa) -- 3.3-2.5 mya

  • Raymond Dart (an anatomist) received an endocast (a solid impression of the inside of the skull) from a site called Taung. Some of the facial bones were also recovered.

  • This became known as the famous Taung baby (or Taung child). The estimated age of this fossil is 3-4 years (based on tooth eruption pattern)

  • He published about this first hominin to be discovered, saying that:

  • The foramen magnum is further forward than for modern apes, but not as much as modern humans

  • Forehead less receding

  • Milk canines were smaller & the new permanent molars were larger, though.

  • But the brain was quite small. Estimate of its adult size at 440 cm3. If you adjust for body size the bonobo is a good comparison.

  • Only later was his discovery well accepted. During much of his own lifetime his interpretation was dismissed.


Later Australopithecus Species/Early Homo 3

  • Three species (continued)

  • Australopithecus africanus (continued):

  • There are three famous fossils:

  • Taung Baby" was found by Raymond Dart in 1924. It was the first hominin found. This was a significant find because it was the first evidence that our origins began in Africa.

  • "Little Foot" was found in Sterkfontein, South Africa by Ron Clark. Clark found this fossil stashed in a box and reassembled it.

  • "Mrs. Ples" was found by Robert Broom. She was conclusive evidence that our origins did indeed begin in Africa. It should also be noted that Mrs. Ples is really Mr. Ples

  • It is generally considered a descendent of A. afarensis and may represent an ancestor of the genus Homo.

  • Debate on who is ancestral to Homo line

  • Johanson and White disagree that this species was ancestral to humans, saying back teeth too specialized.

  • If they are correct this leaves a gap in the fossil record at that time (1979).

  • Australopithecus garhi

  • Australopithecus garhi is found at 2.5 million years ago Ethiopia (garhi means surprise in Afar language)

  • They had a small brain but not a robust anatomy.

  • Both front and back teeth are large, but not as specialized as the robusts.

  • They show evidence of butchering animals with tools, and may be a more likely candidate as ancestor of the genus Homo.


Later Australopithecus Species/Early Homo 4

  • Three species (continued)

  • Australopithecus sediba (welspring/fountain in local Sotho language, ~2 mya)

  • In 2008 two partial skeletons were discovered at Malapa Cave. This is a great story and a significant find. In July 2012, another specimen was found (may be most complete skeleton every).

  • This has been given a new species name as it is a mixture of australopithecine and Homo features.

      • Cranial capacity is estimated at 420 cm3, long arms and curved fingers, and more primitive feet.

      • Short fingers, brain reorganization.

        • Suggests brain reorganization in the frontal lobes may have taken place before expansion of the brain.

        • The pelvis also altered before the brain enlarged; challenges the idea that the changes were due to the larger brain.

        • Many suggest transition species to Homo line.

        • The paleoanthropologist, Lee Berger, suggests Australopithecus africanus evolved into Australopithecus sediba that evolved into the Homo line.

  • No matter which of the three candidates is ancestral, Homo origins date to around 3 mya


Paranthropus 1

  • They went extinct about 1.5 mya.

  • They are close relatives, but not our ancestors. Some suggest a different genus (Paranthropus)

  • Physical traits:

  • The robust name applies to the teeth

  • The front teeth are small, both in size and in comparison to back teeth

  • The premolars and molars were as much as 4X that of moderns.

  • Diet was both soft foods and very hard foods such as nuts, seeds and hard fruits

  • The skull exhibit evidence for heavy chewing:

  • Dished-in face with large cheekbones (zygomatic arch) to accommodate large chewing muscle, the masseter muscle and the temporalis muscle.

  • The masseter runs through the zygomatic arch and connects to the temporal and zygomatic bones.

Temporalis and masseter muscles (humans)

  • The temporalis runs from under the jaw, through the zygomatic arch and attaches to the side of the skull.

  • Sagittal crest (bony crest on top) can develop to aid the anchoring of the temporalis muscle in species with large muscles.

  • The rest of their bodies were not that large, average weight was 100 pounds for males, 70 pounds for females.

  • They were bipedal and had small brains.


Paranthropus 2

  • The three species:

  • Paranthropus aethiopicus is named for Ethiopia. Most famous is the ‘Black Skull’ found at Lake Turkana, Kenya

  • Dates to 2.5 mya, has a cranial capacity of 410 cm3, and is similar to A. afarensis in traits

  • Very robust with primitive cranial traits (such as the anatomy of the base of the skull)

  • Suggested intermediary between A. afarensis and the later Paranthropus species

  • Paranthropus boisei (2.3-1.2 mya) is found in Omo, Ethiopia and Olduvai Gorge, Named after Charles Boise (funded the excavation).

  • Formerly named Zinjanthropus boisei

  • Cranial capacity closer to 510-530 cm3 and facial features are still large and broad

  • Paranthropus robustus is found in South Africa (2.0-1.2 mya), first by Robert Broom

  • Dentition not as robust as E. African members of this genus.

  • Some suggest that Australopithecus robustus is a regional variant of Australopithecus boisei



The database is protected by copyright ©essaydocs.org 2016
send message

    Main page