A Lousy Companion

When it comes to making an educated guess about when early human populations took to dressing themselves, there are many different and valuable approaches. Some lines of evidence are more obvious than others, like inspecting human burials for durable clothing remnants. Other lines are more subtle, such as tracing the gradual change from robust to gracile phalanges on prehistoric feet in places where shoes became commonplace.
While these sorts of methods that look to archaeological excavations yield vast stores of data, the drawbacks of their source still remain: the sites and objects may be dated, but they are only a small sample  in time, and not all types of material survive long enough to be found. Another way to bolster and frame the information we collect comes from the discreet, yet ubiquitous louse.

A fossil specimen alongside a modern Pediculus humanus humanus (head louse)
Source: http://jameselassiter.blogspot.com/2011_04_01_archive.html

Head and body lice as well as pubic lice provide a few hints about human behavior, having thrived alongside humans since the early years of our divergence. Pediculus humanus of the head and body are distinct creatures unto themselves; head lice live primarily on the scalp, while body lice nest in clothes rather than on skin. These varieties of lice comprise a sister species of the chimpanzee's, while pubic lice is actually one that was once shared with early gorillas. Primate lice is extremely dependent on their hosts, so their evolutionary history is entwined with their hosts. Newly analyzed genomes illuminate an adaptive past as lice were forced to migrate and evolve within their human environment.

As the technology to sequence genomes was opening up a wealth of possible analysis, Dr. David Reed of the University of Florida started requesting samples of gorilla lice from various sources to analyze its DNA. What he discovered helps focus the chronology of early human hair reduction, supporting conventional theories on when pre-humans left the forests and where bipedalism may have begun.

The divergence of pubic from gorilla lice likely happened 3.3 million years ago, using the degree of distinction between the two sets of DNA. This general time is also where researchers predict our ancestors lost their bodily hair (Toupe 2010). The bare skin would have been ideal for regulating heat in savanna-like areas, and our bipedal activities would only really require the tops of out head to be protected by hair. This would result in the isolation of our parasitic visitors, left with only head and pubic hair available for habitation (Wade 2007).

This diagram illustrates the movement of lice as humans diverged as a species
Source: http://scienceblogs.com/loom/2007/03/07/question-of-the-day-how-do-you/

As gorilla's lice seem to have invaded the groin area, and the chimpanzee-related lice would have found their refuge on the scalp. According to its evolution, P. humanus spent quite a while on our crowns, letting us assume there were no tailored clothes to move into until a little more than 100,000 years ago. Furs and informal drapery might have been used by people beforehand, but nothing particularly or consistently close to the skin to provide nesting areas. Geneticist Mark Stoneking's analysis of body lice finds the minimum date of it's divergence to be about 107,000 years before the present. From this finding he suggests that sewn clothes would have emerged shortly before (Wade 2007).

Other researchers have published much earlier dates for man's employment of dress as well as the split from chimpanzee ancestors. Entomologists and other geneticists have pegged our emergence (coinciding with the separation of chimpanzee and human lice) between five and seven million years ago. Their estimates of the habituated clothing show it to be a bit older than 190,000 years, though other dates are not entirely discounted. Other experts still place the date for dress before, claiming it came about 650,000 years ago (Harmon 2010). At what times and places clothing transitioned from loose coverings to purposefully crafted articles remains unknown.

This graph presents a possible chronology based on various evidence
Source: http://leherensuge.blogspot.com/2010/09/louse-tmrca-provides-estimate-for-human.html

While it's good to employ a variety of perspectives to tackle such a complex and telling behavior, it's also good to keep in mind that results are prone to vary along with their differing lines of evidence. Dating doesn't often show absolute maximums or minimums of actual trends, but the different analysis between experts here provides a very wide range of possibility. While body lice offer a lot of compelling data, only a couple gene samples have been sequenced. Where does it fit in next to the archaeological record? It does not provide direct evidence of human activities, but can intimate many potentials and confirm many suspicions.

How close are these estimates? How much can lice tell us? Because actual hair and clothing cannot preserve for thousands of years, this is both a highly debatable and very significant source of information.

Sources:

Harmon, K
2010   Full Genome Sequence Shows Body Lice Have Lousy Sense of Smell. Scientific American

 Wade, N.
2007   Head-scratching Puzzle: What Lice Have to Say about Human Evolution. New York Times

Toups, Melissa A., Andrew Kitchen, Jessica E. Light, and David L. Reed.
2010   Origin of Clothing Lice Indicates Early Clothing Use by Anatomically Modern Humans in                              Africa. National Center for Biotechnology Information. U.S. National Library of Medicine