look at mankind, for better or worse, we're pretty special. Arguably no
single species has been able to reshape the planet in its image as fully as
humans. So what happened during the course of evolution to make humans so
dramatically different in intelligence and abilities from even our closest
relatives, say chimpanzees?
That's what a multi-school team set out to answer [press
release], with advanced genetic analysis. The team was led by
David Kingsley [profile],
a top geneticist at the non-profit Howard Hughes
Medical Institute, and included a number of researchers from Stanford University, which is located in Palo
I. The Hunt for Missing Genes
While some find it hard to fathom how a set of small genetic changes could
result in dramatically different anatomies -- say
growing fins versus arms -- Professor Kingsley has shown in stickleback fish
changes to regulatory DNA could have dramatic impact on anatomy and appearance.
Similarly some of the crucial differences between humans and chimps were found
to be not actual gene additions or deletions, but rather changes to regulatory
In total 510 gene sequences can be found in chimps and a variety of other
mammal species, but are "surprisingly missing" from humans.
Computer analysis showed that many of these sequence deletions were
clustered around steroid hormone genes (which influence sex and anatomy) and
neural development genes.
These regulatory changes were thought to grant humans unique traits like bigger
brains and erect spines, and cause them to lose other traits such as whiskers
and bony penises (more on that later).
But locating potentially important deletions only took the team so far.
They then needed to analyze the deletions. Describes Professor
Kingsley, "We had a team of interested graduate students, postdocs, and
developmental biologists poring through this list. It was a fun detective hunt
that led to lots of interesting discussions."
II. Penis "Bones" and More -- Intriguing Findings
One inactivation that produced a critical impact in humans was the removal of
regulators of the gene GADD45g around neural-specific sequences. GADD45g
triggers cell growth. So by disabling this regulatory sequence human
brains were able to grow larger and more dense, allowing the rich cortex and
connecting layers that give rise to intelligence, complex personality, and
advanced motor learning.
Other changes revolved around a special sex-specific receptor.
Did you ever stop to wonder why cats and dogs have whiskers and we don't?
Critics of evolutionary theory have long posed such examples. Well
it turns out humans have the genes to potentially grow whisker-like features,
but one of the deletions inactivates the growth.
Specifically the deletion is the removal of a regulatory sequence for the
androgen receptor, which causes the protein androgen to be produced in that
vicinity. Androgen is responsible for "turning on"
male-specific traits. In this case, knocking out the regulatory sequences
knocked out the "on" switch, leaving the genes for whiskers
permanently inactivated (barring a rare insertion of an androgen regulatory
sequence via future evolution).
A similar male-specific inactivation occurs with the androgen receptor and
genes coding for penis bones.
While whiskers may be familiar material, most don't know that chimps and some
other mammals actually have "bony" barbs in their penises.
These barbs aren't true bones. Rather they are made of keratin, the
hard material found in human fingernails.
The penis "bones" act as a form of natural male enhancement -- though
humans seem to be getting along fine without them. Gill Bejerano [profile], a developmental biologist at Stanford
University in California was responsible for tracking down exactly how humans
lost their penile "bones".
Professor Bejerano says we lost our spiny penises approximately 700,000 years
ago, around the time humans split from Neanderthals. A deletion
of an androgen receptor regulatory sequence was, once again, to blame.
Increased neural development may have given rise, in part to monogamy, which in
turn may have allowed the gene to become non-critical.
Why do chimps and other mammals need bony penises? In nature, competition
is always fierce. Frequently multiple males try to fertilize the same
female over the course of a couple days. The dried secretions from the
first males block entrance to latecomers, in an attempt to increase their
chances of being the one to achieve fertilization.
But with a bony penis in hand, such secretory barriers pose little obstacle.
The bony penis is able to break through such obstructions with virtually
no damage to the precious male member.
In humans such situations are rare, so sex organs have become simpler. In
other words, because of monogamy we don't need our penises to be spiny any
The paper on the changes, including the loss of the penis bones is published [abstract] in the journal Nature,
arguably science's most prestigious peer-reviewed journal.
III. The Tip of the Iceberg
Even with a wealth of clues in hand, delving into these mysteries is daunting
says Professor Bejerano. At time the human genome "can be a huge,
impenetrable mystery" he says, but, "[Researchers are] beginning
to tease out some of the molecular differences that make us human."
A lot more "teasing"/working the data will be necessary if we hope to
fully crack that puzzle, though. The investigation into whiskers, brain
development, and penis bones was only the tip of the iceberg.
Armed with the knowledge of where deleted sequences lie in the human genome,
investigators can now perform additional research (one potential target of
interest: removed female evolutionary mechanisms dealing with spiny penises) to
discover more regulatory differences and how they changed humans and make us
look and act far different from our distant monkey relatives.
Outside of scientific interest, learning about how are brains got big and our
penises less bony doesn't seem to have a lot of practical applications,
but Professor Kingsley explains the methods developed will play a critical role
in "solving" the puzzles of genetic-related diseases such
as arthritis, cancer, malaria, HIV, Alzheimer's, and Parkinson's (some of
which are caused by viruses, but in which genetic dispositions play a major
He states, "It's now possible to begin identifying some of the particular
molecular changes that contribute to the evolution of human traits. We think
that the same sorts of lists and approaches will eventually help illuminate
human disease susceptibilities as well. It's a great time to be studying not
only where we came from, but also how our genetic history shapes many aspects
of current human biology."