Evolution of Life on Earth

Carboniferous period 360 to 286 million years ago

335 MILLION YEARS AGO

In places like Scotland, the area was covered at this time with a thick and very tall forest. Indeed much of the continents of the world were bunched together and were covered with this great forest. Rising over 30 metres in height, the forest provided another level of protection in the environment to help retain enough moisture on the ground for any curious invertebrate animals wanting to venture out onto land.

Scientists believe so much oxygen was being pumped out of the forests that the composition of the atmosphere had changed to the point where it contained around 35 per cent oxygen. Fortunately there was no place on land where the forests could dry out and become flammable enough to burn. There was too much fresh water. And as David Attenborough said in his First Life documentary:

'This had a profound effect on animal life.'

In the insect world, this meant life grew to remarkable sizes. For example, the millipedes we are familiar with today would be more appropriately called metre-pedes as they grew to well over 1.5 metres long. Even dragon flies at this time were literally medium-sized birds skimming across bodies of fresh water on the forest floor.

In fact, the first insects to fly would take place during this moment in the evolution of life on Earth.

315 to 300 MILLION YEARS AGO

Since there were few predators on land, some amphibians began to lay their eggs in the sand or under plant growth.

Then, after a few million years had passed and while the warm conditions prevailed in the early part of the Carboniferous Period, these eggs containing the next generation of offsprings developed a tough outer skin to protect the interior from drying-up and from being easily destroyed by the increasing number of predators appearing on land. Animals which layed eggs of this kind are called Reptiles.

Those animals that remained as amphibians would continue to survive despite the increasing number of reptiles and predators making their mark on land. Some amphibians like the frogs would keep themselves small in size and develop specific skin colour to help camouflage their bodies with the plant life. Other amphibians would develop highly toxic chemicals underneath their brightly-coloured skin called alkaloids as an effective chemical deterrent against being eaten alive by the predators.

290 MILLION YEARS AGO

During a time when great sheets of ice was known to have covered large parts of the supercontinent in the south known as Gondwana land, the first mammal-like reptiles appeared on Earth.

According to the latest genome research, the mammal-like creatures were a little different from other animals in that a rogue and seemingly insignificant "Y-chromosome" suddenly appeared in the male species. Prior to this time, animals had enjoyed a pair of identical "X-chromosomes" with gender determined by simple environmental factors such as temperature (for example, a low enough temperature in the environment would usually result in the birth of more males, and a warmer temperature would normally create more females). Now, the mammals with this new chromosome made the gender determination independent of environmental factors.

As David Page of the Whitehead Institute in Cambridge, Massachusetts, described the Y-chromosome's function:

'It [the Y-chromosome] said, I will no longer respond to these environmental cues; if I am present, the male pathway will be followed.' (1)

So important is this discovery in the genes of mammals that it holds the key to the evolution of the human race.

## UPDATE ##
26 March 2005
A detailed analysis of the X-chromosomes in women compared to the X- and Y-chromosomes of men conducted by Huntington Willard of Duke University in Durham, North Carolina, USA, says women are genetically more complicated than men.

This may seem like stating the obvious since the Y-chromosome is indeed smaller in size than the X-chromosome. Therefore men would have less genetic information than women.

Does this mean that women, being more genetically complicated, are better than men? No. It just means men don't need the extra genetic information to achieve their goal(s).

However, to compensate for this apparent "genetic efficiency" in achieving things, men are also more genetically susceptible to mutations such as radiation penetrating and damaging or changing the genes (could this be how the Y-chromosome was originally created?) as there is no additional X-chromosome to wrap around and protect the X- and Y-chromosomes. This makes men more easily able to evolve over time if the changes are favourable to the opposite sex.

Women, on the other hand, are better in looking after the core genetic code needed by humans to look human and survive by having two X-chromosomes. So should one X-chromosome get damaged, there is a better chance for the other chromosome to carry undamaged information, which can aid later in sex through the production of a quality human egg.

What the study has actually shown is that women do indeed use both X-chromosomes. Before the study, scientists thought only one of the X-chromosomes is made inactive and the other X-chromosome provides all the information needed for women (and men) to build the body and perform the tasks needed to survive and hence achieve certain goal(s) during their lifespan. Now scientists realise active genes are found in both chromosomes.

In the case of men, this gender has to rely on the only X-chromosome for making men look human and able to survive in the same way as women can. But it is the Y-chromosome that help to make men for who they are — men.

This rogue little chromosome helps with building extra muscles, make general changes in appearance to help women become more sexually attracted to men, and increases male sexual drive and sperm production as needed for the human species to survive better and ensure there is sex between the opposite genders (rather than among the same sex).

For women, the extra X-chromosome provides the human species with the necessary in-built checks to ensure the essential genes for defining our species as human remain intact.

Another discovery made in the study is that not all active gene sequences in just one or the other X-chromosome of women are used in men. While men may use some of the same gene sequences as women do in order to be human and perform tasks common to both sexes, women have some 200 to 300 genes in the X-chromosomes used only by women. In other words, there are genes in the X-chromosome uniquely expressed for women relative to men.

In essence, women should be considered as having a different human genone than men. (2)

NOTES

Burne 2001, p.4. This smaller rogue chromosome combined with a single X-chromosome in males compared to the pair of X-chromosomes in women is believed to be important in the evolution of mammals as it allowed both sexes to independently evolve their own specific gender characteristics as required for survival in an otherwise tough and unforgiving early environment, as well as to ensure a high probability of reproduction can take place between the sexes. But what actually determines a mammal to become female or male is depended on the way the brain develops and not just the hormones and genes.
## UPDATE ##
30 October 2006
Does this mean homosexual tendencies are unnatural and will never occur? No.
Before the Y chromosome came on the scene, animals were more likely to look similar. It means having sex can be more of a "hit or miss" affair so to speak especially if pheromones or other hidden biological signals are not present.
When the Y chromosomes became a reality, homosexual tendencies would continue (not fully abated) because there are survival benefits. For example, it can help to control population size of the group when faced with varying levels of food resources. Also some female animals deliberately use same sex behaviours as a means of rewarding and enhancing preferred behaviours as a learning technique.
Far from being an act of confusion and lack of reasoning skills, homosexual tendencies in animals can be a well-thought out solution to various problems.
In humans, homosexuality continues to this day. Its presence is further complicated by excess stresses in modern life and overemphasis on L-brain "male" behaviours which can force the brain to seek a balance to this situation. For example, the probability for the youngest male child in a family of males to show signs of homosexual behaviour is said to be higher than in any other family type.

The Canberra Times: Women on top in battle of sexes. 26 March 2005, p.12.