Case study: 51
Syllabus: Evolutionary Biology
Are Humans Still Evolving?
As asked by most people, the question has two meanings. Perhaps the most common refers to the direction of future human evolution; in other words, how will we look in so many millions of years? As our minds do more and more of our work—and our bodies do less and less—will we eventually be great big heads atop short, spindly bodies? (This is the image we are often given of aliens from more advanced civilizations. Think of Close Encounters of the Third Kind, E.T., or The X-Files.)
The answer to the first meaning of the question is, obviously, who knows? Evolution is so complex, so dependent on multiple, interacting  series of events, that there is really no way of predicting the evolutionary future of any species, especially ours, with its ability to control its behaviour, the environment, and, indeed, its genes through culture. If we could take a time machine back to the Cambrian period 543 million years ago and look at its animal life, made up mostly of primitive arthropods (ancestors of modern insects, spiders, and crustaceans), who would predict that a rare little wormy creature only about 2 inches long, called Pikaia, would be the earliest-known representative of the chordates, the important group of organisms now represented by fish, amphibians, reptiles, birds, and mammals (Gore 1993; Gould 1989)? A second, more sophisticated meaning of the question concerns whether we humans have stopped our evolution by so controlling our environment that natural selection is no longer in operation, that genetic variation is no longer an important factor in reproductive success. There are two parts to the answer. First, as we discussed, there are processes other than natural selection that bring about genetic change from generation to
generation in a species. Our control over our environment certainly won’t halt the processes of mutation, gene flow, and genetic drift.
Indeed, one might argue that we have increased mutation rates through some of our environmental manipulations and that our increasing mobility makes gene flow ever more powerful. So, by its genetic definition, evolution will always be taking place in our species.
But have we buffered ourselves against natural selection? For some genetically based characteristics, yes, we have. Remember that fitness is measured against a particular environment. If, through culture, we change the environment, we then change the adaptive fitness of certain phenotypes and thus of the genes that code for them. If I had lived in, say, Homo erectus times (1.8 million to 100,000 years ago), I’d no doubt be dead by now. If my infected appendix hadn’t killed me (which it would have), my nearsighted- ness would have prevented me from being a very effective hunter or gatherer. Our present environment, however, has available all sorts of techniques and devices to improve one’s eyesight. I wear glasses and see my optometrist once a year, so my poor vision (which, for the sake of the example, we’ll say has a genetic basis) does not put me at any survival or reproductive disadvantage. You can probably think of dozens of other examples.
We have not, however, completely eliminated all relevant genetic variation. There are plenty of
genes for diseases that place severe, or absolute, limits on a person’s ability to reproduce and
thus pass on those genes. Tay-Sachs disease, for example, is lethal well before reproductive age.
Sickle cell anemia lowers reproductive rates in the few individuals who live long enough to reproduce.
And let’s end on a hypothetical note (keeping in mind my precaution about predicting future
evolution). There could well be some genetic variables that will make some difference in reproductive
success in the near future. Suppose there is genetically based variation in humans’ abilities to withstand less-than- optimal air quality or severely crowded living conditions or high levels of noise pollution. As these conditions worsen, it is certainly conceivable that genes for such tolerances will become more frequent as their possessors become less reproductively affected by the modern environment.
Who Are the “Hobbits” from Indonesia?
Case study: 52
Syllabus: Phylogenetic status, characteristics and geographical distribution of the
following
In October 2004 an astonishing fi nd was announced (Brown et al. 2004). A partial human
skeleton, dated to 18,000 ya, was discovered on the Indonesian island of Flores. What makes
this find remarkable is that the skeleton is of an adult female who stood a mere 106 cm tall
(about 3 feet 5 inches) and had an estimated brain size of 380 ml, about the stature and cranial
capacity of Australopithecus afarensis . And yet the physical features seem fairly clearly to assign
the specimen to genus Homo, with particular similarities to Homo erectus.
The discoverers gave the specimen the status of a new species, Homo floresiensis ( Figure
11.41). Since this initial discovery (Morwood et al. 2005), more specimens have been announced—
including arm, wrist, and foot bones of the original skeleton, a mandible from a second
individual, and assorted other bones—from an estimated nine individuals in all. Dates range
from 74,000 (and possibly 95,000) to 12,000 ya.Even for those who propose multiple
species of our genus over the last 2 million years, this is an amazing find, because as far as
we know there have been no humans other than us—fully modern Homo sapiens —on earth
for at least 27,000 years. And for those of us who feel that only one species of Homo has existed, the implications of this find and its interpretation are obvious.
What are we to make of these specimens, referred to in the popular press as Hobbits (a reference to
the small characters in J. R. R. Tolkien’s Lord of the Rings )? Given their body and brain size,
could they be australopithecines, indicating that there were populations of this genus outside of
Africa? Probably not.
Although some authorities say the lack of a chin, features of the pelvis, and body proportions
are australopithecine in nature (Balter 2004; Lieberman 2005), they have phenotypic characteristics
that place them clearly in genus Homo . Moreover, the fossils were found in association
with stone tools and evidence of hunting and possibly fire and cooking. None of these cultural
features are associated with Australopithecus .
And the game hunted was not small; it included pygmy elephants and Komodo dragons (the world’s largest existing lizard). Certainly a high level of cooperation and communication would have been necessary to accomplish such hunting. Moreover, no fossils of australopithecines have been found outside of Africa.
Authorities are divided over the interpretation of these fossils, or, more accurately, over the
original fossil, known as LB1, since it provides the vast majority of the data. Some contend it
was an individual with a pathology, perhaps microcephaly, a genetic form of dwarfism (Eckhardt
2008). Other experts support the claim that LB1 represents a normal human of diminutive
size but of a different species
(Falk et al. 2008).
The recently reported wrist bones are said to be “primitive” and thus indicative of a new species
by some (Tocheri et al. 2007), while others disagree (Eckhardt 2008). Still others have
concluded that because the feet are relatively long and not arched, as in modern humans, the
Flores population represents a new species (Jungers et al. 2008). The discussions over this
issue at professional meetings are at least lively and can get quite heated.

So, there was either one, or several, individuals on Flores during the time range indicated that
suffered from some anomaly, or there was a whole population of diminutive humans. If the latter,
there are two questions:
First, were they a different species? We can’t, of course, experiment to see if they could
interbreed with other human populations, so that will always remain an open question.
Second, where did they come from? Did they descend from Homo erectus and respond to a
phenomenon of dwarfing common to island species with restricted room and resources? Or did
a hominid predating H. erectus get to Flores and evolve a more complex brain with no increase
in size (Aiello 2010)?
Virtual images of the brain of LB1 indicate that it is not a miniaturized modern brain but
more resembles Paranthropus in size and H. erectus in shape (Falk et al. 2005).
The debate continues and, of course, requires more data—specifically, more fossils. But one
thing is certain: genus Homo is a lot more variable than we once imagined. And the story
continues to unfold: on the island of Palau, Micronesia, researchers have found fossils of a
possible small-bodied population with brains within the modern human range (Berger et al.
2008).