This course is about what we can learn from examining the human skeleton, and how we can use this knowledge to reconstruct the lives of people who lived in the past. In archaeology and anthropology, human skeletal remains can provide unique insights into the past and the present; insights that cannot be gained otherwise. These insights are explored in five main themes spread out over five weeks of learning. First, it is shown how age-at-death, sex and stature can be estimated by the close examination of (archaeological) skeletal remains. In subsequent modules it is shown how human bones can provide information about the diseases and injuries that people suffered from and what they ate. Also, it is shown how the human skeleton provides information about the kinds of activities that people engaged in and about how they migrated and moved around their landscapes. In this course, you will examine all aspects of the human skeleton that can provide us with information about these different facets of life. Together we will explore the scientific field that is known as human osteoarchaeology. - Human, because it is about us and our ancestors, - Osteo, because it is about our bones, - Archaeology, because we use this information to better understand the behaviors and events experienced by past people. During the course, you will decipher the clues left behind in the skeletons of past peoples with the methods and techniques that are presented. You may also discover some clues hidden in your own skeleton and what they reveal about the life that you are living. Want to know more? You can take a look at the course trailer here: https://www.youtube.com/watch?v=fJi22TxzpRw
Subadult Age-at-Death Estimation
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来自 Universiteit Leiden 的课程
Osteoarchaeology: The Truth in Our Bones
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从本节课中
Bones to Biography & Demography
Welcome to the first content module of this course! This week is all about introducing you to the primary things that we can learn from nearly all human skeletons. Who are the people who lived in the past? Are they men, women or children? How old did they become, and how tall did they get? We can answer these questions by studying a range of bones contained in the human skeleton. Here, we will be showing you exactly how to do that, and hope you will learn a lot!
与讲师见面
Andrea Waters-RistAssistant Professor
Faculty of Archaeology
Welcome to the first instructional video of module one in the course,
The Truth in Our Bones.
In this video, we will learn about two things.
First, we'll provide a basic overview of how the skeleton and
the teeth form and grow.
And second, we'll show you how we can use this growth process
to estimate the age of death of an individual who was still growing.
You will frequently hear us use the term subadult.
Other osteoarchaeologists use the term nonadult.
And what we mean by this is anyone under the age of about 18 years.
We don't use the term child or juvenile for
this, because those terms have a more specific meaning in many fields.
For example, by one scheme, there are five age category within subadulthood.
The fetal period occurring obviously before birth.
Infancy occurring from birth to two years of age.
Childhood from three to six years.
The juvenile period from seven to 12 years of age, and
adolescent from 13 to 17 years.
After the age of 18, skeletal growth is minimal, being completed by your mid 20s.
And you are categorized, physically at least, as an adult.
The methods we use to estimate age in adults,
are very different from those that we use in subadults, as you will see.
There are two ways in which our bones form and grow.
The first is called endochondral ossification, and is how most of
your bones develop, including the long bones such as those in your arms and legs.
The bones or
bone sections that grow via endochondral ossification are depicted in this figure.
Endochondral growth endo meaning within, and
chondral meaning cartilage, is basically growth occurring within a cartilage model.
In utero, this begins with a hyaline cartilage model,
whose shape resembles a small version of the bone to be formed.
The first bone formed is along the bone's shaft called the diaphysis.
Here, bone cells deposit unmineralized bone called osteoid.
This area in the diaphysis is termed the primary ossification center, and secondary
ossification centers occur at the ends of the bone, and are called the epiphyses.
There's a growing cartilage layer between the two areas, a point called
the metaphysis, and this is where osteoid is deposited as a bone grows in length.
Eventually at an age specific for each ephiphysis,
osteoid production of the metaphyisis will outpace the rate of cartilage production,
causing the two to fuse.
So, how can we use our knowledge of endochondral growth to estimate
the when someone died?
We'll tell you about three major ways this works.
First, did you know a newborn infant has more bony elements than an adult?
A new born has around 300 bony elements.
An adult usually has 206.
This is because in subadults,
the separate pieces of many bones have not yet fused together.
Also in subadults, some bones don't form until later.
Babies have cartilaginous precursors of many bones
such as the kneecap called the patella.
And it's not until the age of three to five years,
that the kneecaps turn into bone.
In fact, a four year old has more bony elements than a newborn,
because those cartilaginous precursors have by then started to ossify.
Have you ever noticed that it's actually quite painful to crawl on your knees?
Well, not so much for an infant, without a bony patella in the way.
So, by observing which bones are present in the subadult skeleton,
we can get an idea of how old they must have been.
Second, we can look at the state of fusion of the epiphyses.
So, each epiphysis fuses at a predetermined age.
Now there are some differences within a population, between different geographic
groups, between boys and girls, especially once puberty hits.
But in normal conditions,
all humans will have an epiphysis fuse within the same one to two year period.
For an example, we can look at the upper arm bone, the humerus.
The head of this bone forms a part of the shoulder joint, and typically in girls,
that epiphysis fuses between 15 to 17 years of age, and in boys,
it's typically 16 to 18 years.
So by looking at the state of fusion of different epiphyses for all of the bones,
we can estimate a subadult's age with a margin of error of maybe one to two years.
Finally, the third way of estimating the age of a growing subadult
is by measuring the size of their bones, and
comparing that to measurements of subadults from modern times of known age.
Let's take an example from your upper leg bone, the femur.
Now, obviously in normal circumstances,
as one gets older the size of that bone increases as one gets taller.
We then compare the length of that bone to modern subadults, and
find the closest match.
When doing this from many bones from the skeleton,
we can arrive at an age of death that's accurate to within one to two years.
If you'd like to watch an optional video of how this works,
look under course documents, and Christine will demonstrate the process for you.
Now, let's take a step back and recall that there were two types of bone growth.
We've learned about endochondral bone growth, what about this other type?
You may have noticed, we've not yet talked about the skull.
That's because most of the bones of the skull, especially the thin bones making up
the vaults and part of the face, are formed by intramembranous ossification.
As the name implies, this means that bone forms within, or intra a membrane.
This process does not involve cartilage.
The bones or sections of bone that form via intramembranous
ossification are depicted in the speaker.
So, what happens?
Connective tissue forms in sheets at sites where flat bones,
the intramembranous bones will eventually be.
Some of the cells in the connective tissue will differentiate into osteoblasts
which you recall lay down unmineralized by bone called osteoid.
These osteoblasts begin to lay down spicules in the middle
of the connective layer.
These eventually connect to each other to form a network of trabeculae,
creating a type of bone called cancellous, or spongy bone.
As the osteoblasts continue to lay down osteoid, eventually bone
begins to accumulate along the outer and inner edges, or plates of the membrane.
This bone is much more dense and flat, and it's called compact bone.
So essentially, this process creates a sandwich.
Two layers of compact bone forming the bread, and
the spongy bone in the middle as the filling.
So, what can we do with bones that form via intramembranous growth,
to estimate the age and death of a subadult?
We'll explain how one such method works.
You're probably familiar with the soft spots on a baby's head.
Well, these are real areas on the skull of a newborn or an infant.
And they're called fontanelles.
It's where bone has not yet
formed, instead, there's a tough, fibrous material in that spot.
Fontanelleles enable the bony plates of the skull to flex during birth, and
also facilitate fast growth of the cranium during infancy.
But just as the epiphyses have set ages during which they fuse,
the fontanelles have set ages during which they close over.
As seen in this figure,
some of the smaller ones close over within a few weeks after birth.
The larger one on the back of the cranium closes around one to three months of age,
and the largest one on the front top of the cranium is the last to fully close,
by 18 to 24 months.
So this gives us a fourth way to estimate the age of a subadult.
If we have the skull of a fetus or an infant,
we can use the state of fontanelle closure to estimate its age.
Finally, we talk briefly about dental growth, called odontogenesis.
To understand how teeth grow, it's necessary to learn about their different
parts, the different tissues they are composed of.
This diagram shows the hard outer crown layer called the enamel,
the inner, softer crown layer, called dentin.
The root of the tooth, here surrounded by a thin layer called cementum, and
the pulp cavity, which exists inside the tooth and
allows connection of the tooth to the body's nerve and blood supply.
A tooth first begins forming in the bone of the jaws.
Undergoing four developmental stages.
Your 20 baby teeth called deciduous teeth,
start forming by the sixth to eighth week in the utero.
And your 32 adult or permanent teeth begin forming by the 20th week in utero.
Now, obviously these tiny forming tooth buds don't erupt until much later in life.
But nonetheless they are there, hiding in your jaws, sometimes for many, many years.
So, teeth slowly increase in size in an orderly fashion.
The tooth crown develops from the bottom to the top, and then adds layers
outwardly, whereas the root gradually gets longer in the opposite direction.
The ordered progression of tooth formation with age has been observed on many
known age individuals, and this makes it possible to take a tooth from
an individual of unknown age and compare it to a standard to estimate their age.
As you probably already know, teeth erupt through the bone gums at certain ages.
This figure shows the average age of eruption of the baby, the deciduous teeth.
And similar figures to exist for
the average age of eruption of the adult permanent teeth.
So, we can look at the pattern of dental eruption of a subadult, and
compare that with standards of eruption for
similar geographic groups, to estimate how old someone was when they died.
Dental formation and eruption are less affected
by environmental disruptions such as malnutrition and illness.
So, actually they're our most accurate and
most precise method of subadult age estimation that we have.
If you want to see Anne-Marijn estimate the age of a subadult based upon the state of
their dental formation and eruption, go to a video under course documents.
In this video, you've had a crash course in skeletal and dental growth, and
in the process learned how we have several ways listed here to use these processes
to estimate the age-at-death of a subadult of unknown age from an archaeological site.
In the online forum, you'll find lots of topics for
conversation that we didn't have time for in this video.
A big focus of these questions is on considering what
factors can cause disagreement between someone's actual chronological age,
and the age suggested by their bones and teeth.
As we went through this video, I'll bet that you already thought of reasons why
our methods, sometimes at least, may not work that well.
And this is a big area of study in osteoarchaeology.
As we strive to use methods that are as accurate as possible.
So please join in the discussion.
Coming up next,
is how we estimate the age-at-death of adults for whom growth has ceased.
See you there.
