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Case Study 15. Reading the Bones (3): Tracking Life History at Nariokotome

Case Study 15. Reading the Bones (3): Tracking Life History at Nariokotome

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Case Study 15. Reading the Bones (3): Tracking Life History at Nariokotome



Determining the developmental age of a skeleton depends on identifying changes

that occur at predictable rates or ages. One of the most reliable sequences of changes

for nonadult individuals involves the development and eruption of teeth. Crown

formation, root formation, and eruption occur for each tooth in a regular pattern. All

may be readily detected on X-ray images. Fortunately, all of the Nariokotome boy’s

teeth are present, except for the unformed third molars, or wisdom teeth. The upper

permanent canines have not erupted and the deciduous canines are still in place.

Most of the teeth, however, were not completely formed, as the roots were still

growing. One of the upper third molars is visible on X-ray still within the bone. It is

therefore possible to present an independent estimate of developmental age for each

tooth, based on modern human standards. Those estimates will vary depending on

the human population to which the specimen is compared. B. Holly Smith has

assembled this data and evaluated the fossil. Part of her analysis is presented in

Table 1 comparing the fossil to one of her comparison groups (North American

white males) and also to great apes. Most of the teeth indicate a developmental age

of 10–11 years. Using other reference populations or patterns of dental maturation

does not alter the results substantially.

Table 1 Estimation of dental age of the Nariokotome fossil on the basis of human and great ape

samples (Smith 1993)

Tooth

Maxilla

I1

I

C1

P3

P4

M1

M2

M3

Mandible

I1

I2

C1

P3

P4

M1

M2

M3

Average dental age



Age on human

scale (years)



Age on great

ape scale



Root fully developed

Root length complete, apex not

closed

Root length two-thirds complete

Root length two-thirds complete

Root length three quarters

complete



At least 10.6

10.1



At least 6.5

6.2



9.5

9.9

10.6



8.2

6.6

7.0



Root length two-thirds complete

Crown incomplete



11.4

12.3



6.6

6.7



Root fully developed

Root fully developed

Root length three quarters

complete

Root length half to two-thirds

complete

Root length half to two-thirds

complete

Root fully developed

Root length half complete

Crown incomplete



At least 9.2

At least 9.9

10.2



At least 6.5

At least 6.7

8.6



10.0



6.4



10.5



6.6



At least 10.0

12.3

10.7

10.7



At least 5.7

6.2

6.7

6.9



Development in KNM-ER 15000



The Age of Nariokotome Boy



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Anthropologists must be cautious in evaluating the fossil by a modern human

scale. Humans do mature more slowly than do other primates, so this is likely to

produce an upper limit on age. In one obvious way, the Nariokotome boy differs

from modern people. Normally the modern human canine erupts a year or so before

the molar, but in the fossil, the upper second molar has already erupted, while the

deciduous upper canine has not yet been lost. The fossil closely reflects the eruption

sequence seen in apes, where the much larger canine teeth take longer to develop.

Thus, the second molar comes in earlier (at least 7.5 years) and the canine much

later. Using more comprehensive data from the teeth, Smith estimated the

Nariokotome specimen to have a chimpanzee dental age of 6–7 years based on the

molars and other teeth, but more than eight according to the canines. These figures

define a likely lower age limit.

Another means of determining developmental progress is to examine the fusion

of elements of the bones. Most of the bones of the body are first created from cartilage, which is a softer embryonic tissue more capable of growth. Within that cartilage, one or more centers of ossification will appear where the cartilage degenerates

and is replaced by bone. The centers of ossification expand until they replace all the

cartilage.

Long bones of the body typically have at least three centers of ossification. One

begins in the middle of the shaft. Usually the joint surface on each end, called an

epiphysis, ossifies separately so that the joint is supported by strong bone from an

early age. As a child grows older, those ossification centers expand toward one

another. The cartilage between them may continue to grow, adding length to the

bone. Cartilage growth is stimulated by growth hormone, while ossification is accelerated by sex hormones. As the child enters adolescence, there is a surge of growth

hormone, corresponding to a rapid increase in height. Puberty is caused by a greater

release of sex hormones, and the spreading ossification centers begin to overtake the

growing cartilage. Growth ceases when the ossification centers meet and the epiphyses fuse to the shafts of the bone. Body height stops increasing about age 18–20,

earlier for girls than for boys. The timing of epiphyseal fusion will vary for different

bones. Various factors can alter growth rates. For example, malnutrition may not

allow tissues to respond vigorously to growth hormone, leading to smaller stature

by the time growth ceases. Good nutrition may permit a person to achieve maximal

growth. Hypernutrition, especially a steady surplus of calories, may cause rapid

growth and tall stature, especially in childhood, but it may also facilitate an early

puberty and thus an early cessation of growth. These factors introduce some uncertainty into aging a skeleton.

The Nariokotome skeleton has bones with centers of ossification in varying

degrees of development. Each of these provides an independent comparison to the

modern human pattern. For example, the three primary units of the coxal bone in the

pelvis—the ilium, ischium, and pubis—have not yet fused. Fusion of the coxal bone

normally begins around age 9–12 and is completed around age 14–18. Some of the

elements of the humerus have fused; others had not. This would place the

Nariokotome child between 12–15 and 14–16 years. Overall, using such information, Smith estimated a skeletal age for the fossil at about 13–13.5 years with some



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Case Study 15. Reading the Bones (3): Tracking Life History at Nariokotome



uncertainty. However, when chimpanzees were used as a reference, she calculated a

skeletal age of about 7.5 years.

When the results of dental and skeletal studies are combined, there is some discrepancy between them on the human scale. The skeleton appears more advanced

than the teeth. The chimpanzee scale appears to produce more consistent results

comparing the overall dental and skeletal ages but runs into greater problems with

internal correlations. The age estimates from the canines are greatly out of line with

those from other teeth. This is to be expected because the canines of early Homo are

already reduced in size and thus take less time to develop. We are left with the

unsurprising conclusion that neither model fits the fossil perfectly. Instead, the fossil fits well as one point in an evolutionary spectrum that connects chimps and

humans. More specifically, the developmental schedule fits between humans and

what we know of Australopithecus development.



Pinning Down the Rate of Development

Understanding the absolute age of Nariokotome boy is particularly important

because humans and chimpanzees develop at substantially different rates. A debate

had already been raging over whether australopithecines showed a chimp-like rapid

maturation or a human-like slow one. The Nariokotome skeleton provided fresh

evidence on that question and on the related issue of when the evolutionary change

occurred. To answer that question of maturation rate, one not only needs to know

the developmental age of the fossil, but also its absolute chronological age at the

time of death. The tools for determining this were developed in the decade after the

discovery.

Dental enamel is laid down in a daily cycle during the period of tooth crown

formation. Daily deposits can be observed under an electron microscope as striations on the enamel. It is possible to count them as one would count rings on a tree

and thus to compare enamel formation times of living and fossil species. Hominins

and apes differ in enamel thickness, but this difference is independent of the differences in rate. Apes deposit enamel faster and they complete crown formation more

quickly.

Chris Dean and his colleagues applied this technique to examining fossil hominins. The australopithecine and early Homo specimens they examined laid down

enamel as slowly as African apes, but took somewhat longer simply because the

final enamel thickness was greater. For example, the Nariokotome specimen completed occlusal enamel deposition on the upper medial incisor in about 212 days,

while modern humans take over 289 days, a third again longer. Other teeth show

similar differences. Although the pattern of skeletal development for Nariokotome

may resemble modern humans in many ways, the absolute rate was much faster. In

chronological age, the fossil was probably 8 years old.



Questions for Discussion



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How fast an individual grows and how long he or she lives is the result of

many evolutionary trade-offs. There are reasons to grow up quickly. The faster an

individual matures, the sooner he or she can begin to reproduce, and there is

definitely an advantage to getting a step ahead of the competition. The probability

of dying before getting a chance to reproduce—because of disease, a fatal accident,

or being eaten by a predator—increases the longer one puts off puberty. On the

other hand, maturing slowly gives one time to grow bigger. Bigger may mean one is

safer from predators or more likely to succeed in the all-important competition for

a mate. Maturing slowly gives one more time to set aside energy and nutrient

reserves to spend on the next generation.

Humans grow slowly. Our large brain is consistent with delayed maturation and

a long life. We do have a few unique aspects, though. Our gestation period is less

than we might predict from our brain size—9 months versus 18 months, according

to one estimate. We wean our children early—as late as 5 years in some populations,

but more typically 2 or 3 years in nonindustrialized populations. As a result, all

humans all spend a period of their lives when they are not nursing, but are still

totally dependent on adults for food and survival. Childhood defined in this way is

unique among animals. It has profound implications for our social organization and

economy, since parents have to invest in their children much longer and more than

one adult is needed to provide for a child. However, it ultimately increases fertility.

The mother potentially can start her next child sooner and have overlapping dependent children.

When did life history change to the modern pattern? If the Nariokotome boy

matured at the rate indicated by his bones and teeth, he likely did not experience

much of a childhood. Although the brain was getting larger in early Homo, the

human pattern of slow development arrived much later.



Questions for Discussion

Q1: How does one tell that a mammalian skeleton comes form an immature

animal?

Q2: The upper canine tooth was the last to be replaced in the Nariokotome boy and

was the most troublesome in determining the dental age. What is unusual about

the canine in human evolution that would explain this?

Q3: Why do species have so many different life history strategies? Why isn’t there

one best strategy?

Q4: What are the costs and benefits of having a large brain? Why do we have one?

Why don’t more species?

Q5: What are the costs and benefits of childhood from the perspective of the child?

of the mother?



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Case Study 15. Reading the Bones (3): Tracking Life History at Nariokotome



Additional Reading

Dean MD (2006) Tooth microstructure tracks the pace of human life-history evolution. Proc Biol

Sci 273:2799–2808

Dean MC et al (2001) Growth processes in teeth distinguish modern humans from Homo erectus

and earlier hominins. Nature 414:628–631

Smith BH (1993) The physiological age of KNM-WT 15000. In: Walker A, Leakey R (eds) The

Nariokotome Homo erectus skeleton. Harvard University Press, Cambridge, pp 195–220

Walker A, Leakey R (eds) (1993) The Nariokotome Homo erectus skeleton. Harvard University

Press, Cambridge

Walker A, Shipman P (1996) The wisdom of bones: in search of human origins. Weidenfeld and

Nicolson, London



Case Study 16. Democratizing Homo naledi:

A New Model for Fossil Hominin Studies



Abstract Analysis of hominin fossils generally requires access to the original

material, but that lies scattered among museums around the world. New finds

may sit for years inaccessible to scholars before they are formally published. Lee

Berger has challenged this convention with his discoveries of two new species,

Australopithecus sediba and Homo naledi, which he has made available to the field

with rapid publication involving teams of both senior and junior scientists. These

two species near the transition to genus Homo, join a series of recently recovered

hominin skeletons. Each of them might tell us about the origin of humans, but each

one seems to tell a different version of the story.



It has long been an ironic joke that there are more paleoanthropologists than fossil

hominins. Disregarding isolated teeth, unaffiliated postcranial bones, and fragments, that remains close to the truth. Casts may or may not be available for purchase, but they are commonly expensive. Studying the real fossils requires travel

around the globe, so that time and money become serious barriers for most anthropologists. It is unfortunate that they may face additional institutional or political

obstacles to access.



The Closed World of New Hominin Fossils

When a new hominin fossil is discovered, by common convention it is the privilege of the finder to publish a description. Until that time, even if there has been a

press release, other scholars are usually not permitted formal access to study or

publish on it. Commonly, the finder releases an initial study. After that, if the fossil

is important, the discoverer may decide who should perform a more comprehensive study. He (rarely she) may take that task on himself or delegate it to a colleague or student. A detailed description, analysis, comparative study, and

evolutionary assessment usually require a number of years, and some descriptions



© Springer International Publishing Switzerland 2016

J.H. Langdon, The Science of Human Evolution,

DOI 10.1007/978-3-319-41585-7_16



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Case Study 16. Democratizing Homo naledi: A New Model for Fossil Hominin Studies



have never been published. Only then is the fossil fair game for legitimate scholars

to examine and write about. Practice varies, though most institutions are reasonably generous at this stage.

There is much to be said for this system. Each new fossil may add to or rewrite

the sparse existing record. Ideally, large parts of our collections should be reassessed from the start with each major addition; but that is rarely practical. A

thorough study establishes a record for all to consult and critique, although tracking down often obscure monographs, cost, and language may still constitute

hurdles.

When Ardipithecus ramidus was first reported and named in 1994, it was a crucial find that extended the hominin fossil record 800,000 years further into the past,

to about 4.4 million years ago. Ardipithecus was purported to be close to the ancestral line of the australopithecines and, ultimately, ourselves. As the initial material

was announced in Nature, the team of anthropologists responsible for it, led by Tim

White, made an even more impressive discovery of a partial skeleton. The bones

were so fragile, they had to be protected in a plaster shell before they could be

extracted from the sediments and taken to the laboratory in that condition. Such old

fossils potentially could shed light on the origin of the hominin lineage and the initial evolution of such distinctive human traits as bipedalism. Anthropologists waited

impatiently while the recovery and stabilization of the fossils proceeded slowly. The

crushed skull and pelvis proved so fragile that virtual images of the fragments were

created and manipulated on a computer to reassemble them.

As White’s team conducted a thorough comparative and functional analysis of

Ardipithecus, new discoveries were made. Orrorin (in 2001) and Sahelanthropus (in

2002), both at least 6.0 Ma displaced Ardipithecus as the oldest hominin, and the

partial femora of Orrorin showed evidence of bipedalism. The still undescribed

skeleton threatened to be an anticlimax.

White’s team finally published their preliminary analysis in 2009, 15 years

after its discovery. Their reconstruction was a great surprise and suggested that

either the last common ancestor of humans and apes was not at all the chimp-like

climber that was expected or that Ardipithecus was more distant from human

ancestry than White claimed. As of this writing (2016), a more detailed study is

ongoing and outside researchers have not been able to provide independent

assessments of the original material. Although the Ardipithecus fossils present

unusual problems, the access restrictions are not uncommon. For example, the

partial skeletons from Dmanisi, described initially in 2007, have not been made

accessible to outside researchers.



A New Business Model

The South African paleontologist Lee Berger has made a concerted effort to

change this practice. His first chance came in 2008 when with his son he discovered australopithecine remains in Malapa Cave in the Cradle of Humanity region



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Case Study 15. Reading the Bones (3): Tracking Life History at Nariokotome

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