First (Human) Love

Simple. This week, write about your first love. Maybe it was just a crush, unrequited and fleeting, or maybe it was a tragically epic romance set in a schoolyard. Whichever it was, make sure to put your heart into this one.

When kids reach a certain age they begin to wonder where they came from.  I imagine the conversation that follows often starts with “Well, when a Mommy and a Daddy really love each other…”. But where did Mommy and Daddy come from?

“Well, when a Grandma and a Grandpa really love each other…”.  But where did Grandma and Grandpa come from?

“Well, when a Great-Grandma and a Great-Grandpa really love each other…”.  You see where this is going.

If you trace your family tree to its tallest limbs, who is at the top?  Who came before Angus McFergus, the Scotsman who came over to Canada in 1834?  Before Amelia Everly, the handmaiden to Mary Queen of Scots, and Jean Bertrand, the French seaman who found himself in a London port in 1462?

One interesting stop on the route to the tip of the tree for a lot of people is Genghis Khan.  0.5% of the world’s population (8% of those living in Central Asia) have genetic markers associated with the 13th century Mongolian warlord.  He and his kin were very powerful and had many wives who bore many children.  Talk about leaving your mark on the world.

If we keep climbing those branches, eventually we move past the point where writing was invented (~10 000 years ago) and so we lose historical records.  We must instead rely on the records kept in our cells to identify not individual ancestors, but groups.  What kind of records do our cells keep, though?

Time for a little introductory biology lesson.  Deoxyribonucleic acid (DNA) is the main long-term information storage molecule for the vast majority of life on this planet.  It lives, tightly woven, in the control centres (nuclei) of our cells.  It comes in dense packets called chromosomes and there are 24 different types of human chromosome.  Conveniently, biologists have given the chromosomes pretty easy names to remember.  The first 22 are just numbered 1-22 and all humans, male or female, share these.  The last two are called X and Y and they determine a person’s sex.  If someone’s cells have two Xs, they are female.  If there is one X and one Y, they are male.  This will be come a little more clear as we move along.

Body (somatic) cells, the ones that make up your arms, legs, stomach, pituitary gland, and most of the rest of your body, have two copies of every chromosome.  We call this being diploid.  So if you looked at the nucleus of skin cell under a microscope, you should be able to count 46 packets of DNA.  This is true of the vast majority of cells in your body.  The one exception is in your gonads: they produce special cells called gametes that are haploid, meaning they only have one copy of each chromosome.  While you may not see the word gamete very often, I am confident that you are familiar with the common names of human gametes: sperm and eggs.  When two haploid (23 chromosomes) gametes really love each other, they make a diploid (46 chromosomes) zygote.  This zygote will develop into an embryo and will eventually emerge from the womb and celebrate the anniversary of the most traumatizing event of their life with cake.

So, with that bit of background information in place, we can get to the interesting part: where do we come from and are we all related to Adam and Eve?

One of the great accomplishments of the last twenty years has been to complete the Human Genome Project (HGP), a map of every gene (section of DNA that codes for a particular trait) that humans can possess.  This incredible feat of human ingenuity and cooperation was completed by an international team in 2003 but work continues on it to this day.  With the data from the HGP, some researchers have begun to answer the question of human origins by teasing out patterns in the DNA of the current population.

While many of these genetic studies are fascinating, what I want to talk about today is the human karyotype.  A karyotype is simply the full set of chromosomes and is best illustrated with an image.

Notice how there are 23 sets of chromosomes. Courtesy: National Human Genome Research Institute
A human karyotype.  Notice how there are 23 sets of chromosomes (46 individual chromosomes).
Courtesy: National Human Genome Research Institute

Our closest primate relative is the chimpanzee.  The human lineage (Homo) was estimated to have diverged from the chimpanzee lineage (Pan) about 6 million years ago.  The next closest relative is the gorilla.  The human/chimpanzee line diverged from gorilla (Gorilla) about 7 million years ago.  It is very interesting to notes that both chimpanzees and gorillas have 48 chromosomes, where humans only have 46.  It is much simpler to assume we lost a set along the way rather than two species gaining one.

So where did these missing chromosomes go?

It turns out if you look at the chimpanzee chromosomes, two of theirs look suspiciously like the top and bottom parts of our chromosome 2.  This led researchers to examine human chromosome 2 a little more carefully.  Upon inspection, they found convincing evidence that chromosome 2 was made from the fusion of two ancestral chromosomes.  There are end-markers in the middle and an extra centromere (the central pinching point) in human chromosome 2.

This means that the last common ancestor of humans and chimpanzees had 48 chromosomes.  This is intriguing, because chromosome numbers are very important.  If you have one too many or one too few of the vast majority of chromosomes, you will not survive.  One of the only non-lethal aneuploidies (a chromosome number that is not 46) is three copies of chromosome 21: Down’s Syndrome.  An extra copy of pretty much any other chromosome will kill you.

So how did we get from 48 to 46?

One unlucky ancestor had two of their chromosomes fuse in their sperm or egg.  This means that instead of the then-normal 24 chromosomes, this gamete had 23.  But if you are the only one with 23-chromosome gametes, that means any babies you have will have 47 chromosomes.  Somehow one of these 47-chromosome miracle babies must have survived long enough to reproduce.  This individual would create some gametes that have 24 and some gametes that have 23.

Remember that we are trying to get two 23-chromosome gametes together so that we can have a 46-chromosome human.  The chances of several individuals randomly mutating the same chromosome fusion in the same place at roughly the same time are very low.  This means that when the 47-chromosome miracle baby grows up and is looking for a mate, it can pretty much only choose one with 48-chromosomes.  This would create some then-normal 48-chromosomers, and some new-fangled 47-chromosomers.  But it’s impossible to get to 46 this way.  In order to get to 46, two 47-chromosomers need to get together.  But there were so few 47-chromosomers!  And all the ones that did exist were related!

That’s right, ladies and gentleman, in order for humans to exist as we know them, someone way back at the top of that tree pulled a Luke & Leia.

And that act created the first human.

Aren’t you glad you read until the end?

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