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Topics
As
a paleoanthropologist, I am most interested in the
evolution of human
morphological variation, and how different mechanisms
(such as
taxonomy, sex,
age, and time) explain what is observed in fossil
data. This question
provides a crucial link to the core of
anthropology,
“What makes us human?” Biological
anthropology
is in a unique position to integrate biology and
culture,
and to
bring together sciences and humanities.
This
is reflected in my research so far.
A
significant number of questions in paleoanthropology
deal with
variation and
species: Is there too much variation for this fossil
sample to belong
to a
single species? Does the
new fossil
discovery justify a new species?
These
questions are fundamentally about the nature and
pattern of
morphological
variation in fossil samples. One
could
start from the position that variation calls for
taxonomy; hence,
a new
species is proposed when fossils show some difference. However, I subscribe to an
opposite position that
variation within
species should be examined before taxonomic
explanation is invoked. My
research has shown for several fossil
samples that
there is not too
much variation to invoke an explanation of multiple
species: the
habilines (Homo
habilis and Homo rudolfensis) (Lee and
Wolpoff 2005); Homo
erectus (or, Homo georgicus) fossils
from
Dmanisi, Georgia (Lee 2005); and Neandertals and
modern humans (Wolpoff and Lee
2001; Ahern, Hawks and Lee 2005).
Sexual
dimorphism (morphological differences between two
sexes) is one of the
major factors of variation, and its evolutionary
changes is the one
topic that I have continued since my doctoral thesis (Lee 1999)
and developed further. Studies of sexual dimorphism
in fossil samples
are hampered by a simple fact that sex is almost
always unknown for
fragmentary fossil specimens. I
developed
a method (named “ARM” for Assigned Resampling Method)
that estimates
the degree of sexual dimorphism in samples of unknown
sex, and
demonstrated that the method was reliable (Lee 2001)
. Using
ARM,
I examined the pattern of changes in the sexual
dimorphism of the
hominid fossil record and concluded that human sexual
dimorphism has
not changed for a long period of time.
What
this conclusion implied was that the modern human
pattern of sexual
dimorphism has a long history, at least as long as 1
million years.
Age:
When and How Did Humans Get Old?
Modern
humans are unique in having a long life-span and a
high proportion of
old people
in a population. However, due to the difficulty
of estimating an
accurate
age-at-death in skeletal remains, this question has
not been examined
empirically. proposed
that longevity be
defined broadly, as the proportion of old
adults relative to young adults (named “OY ratio” for
Old/Young ratio),
which can then be analyzed on a large data set of
human fossil
materials.
Results showed that longevity increased over
time with
statistical
significance, but that the greatest increase occurred
with the Upper
Paleolithic, 30,000 years ago (Caspari and Lee
2004)
. This time period is
associated with a
burst of artistic and
symbolic behaviors that have traditionally been
characterized as
uniquely modern
human. The results imply
a
selective advantage in having older adults in a
society, in forms of
information
transmission from generation to generation. Although
this idea is not new, our research was groundbreaking
because we
defined “longevity” in ways that allow an empirical
exploration of the
subject
.
Evolutionary
studies inherently deal with changes over time. However,
studying morphological changes over time poses a
statistical
problem: the data points along a temporal sequence are
not random, but
directional. Although there
are
statistical methods that deal with such relationships,
they are of
limited use
in the study of fossils because fossils almost never
meet the criteria
for valid
statistical analysis. In
addition,
those methods may not provide information about
non-linear patterns of
change.
For example, we know without question that brain
size increased
over time
in human evolution, and that the increase is
statistically significant
. The question remains
whether it changed
gradually or
suddenly. Our research showed that
there
is no evidence that there was a punctuated change in the
pattern
of
increase in hominid brain size since 2 million years ago (Lee and Wolpoff
2003).
Interpretation
of the biological meaning of differences between
samples is a classic
dilemma in
paleontology: are differences between earlier and
later fossils due to
evolutionary change within a lineage, due to
replacement of an earlier
species
by a later one, or can the differences be attributed
to sampling
bias? For
example, Vindija and Krapina are Neandertal sites that
differ in time. Differences
among the fossil specimens from
the two sites are
undeniable. An early
sample might
have a higher proportion of females and juveniles than
a later sample
from an
evolving lineage. We
showed that it
is quite unlikely that the two samples came from the
same population (Ahern, Lee, and Hawks 2002).
What
can DNA tell us about our
origins?
Toward
the end of my graduate training, I became interested
in population
genetics (Hawks, Hunley, Lee, and Wolpoff 2000). Upon
completing my doctoral program, I joined the
laboratory of N
Takahata at the Graduate University for Advanced
Studies (Sokendai) in
Hayama, Japan, as a
post-doctoral fellow.
As
a human paleontologist in a molecular evolution
laboratory, I was a
part of an interdisciplinary effort to understand
modern human origins.
Our examination of DNA
sequence data showed the range of
conditions under which the multiregional model of
modern human origins would be feasible (Takahata,
Satta, and Lee 2001). The likelihood of
realizing those
conditions was
quite small, which some people interpreted to mean
that multiregional
evolution
is extremely unlikely to have happened. To the
contrary, it is
why we
don't see many other species like humans.
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