[MarianaM (OP)]

Paul is asking...

Ancestry.com tells me that from my DNA I have in excess of 49,000 connections. How accurate are these DNA tests?
Also, if the implication is that years go the world had a massive population, but I am sure that is not the case, and if there were an "Adam and Eve" beginning, then my family tree must be shaped like a rugby football (if only considering my direct ancestors and not the siblings) and the implication there is that cousins were marrying cousins.
The next interesting consideration is "Where is the equator of my family tree?".  Perhaps my analogy of a rugby ball is incorrect and my family tree, if ever completed to my generation might resemble a long inflated child's balloon, or some distorted durian, or a banana with cancer.

Any thoughts?

[chiralSPO]

Well... I doubt there is any validity to the "Adam and Eve" scenario.

As I understand it, the emergence of a new species is not usually because there are two mutants of one species that happen to be different enough to be its own species, and then they go off together and start their own species.

Instead, there are populations that are isolated from each other and then over the course of many generations, the natural mutations and different selective pressures can cause some differentiation between the populations. If this goes on long enough, the two can become genetically incompatible, and then we define them as different species.

We can see a little bit of this when considering differences between people today: https://en.wikipedia.org/wiki/Recent_human_evolution

There appears to be evidence that hominids interbred multiple times before homo sapiens wiped out the rest--some folks today have some neanderthal genes!

Back to the shape of the tree...
I think you may also be surprised by how many of those branches come back in. While we consider relationships between siblings (sharing both parents) or between half siblings (sharing one parent) to be incestuous today, I'm sure it happened historically to some extent. More common (and still acceptable in some places today) are relationships between cousins (sharing one set of grandparents), and this was more more common a long time ago (especially in very small and isolated populations). The longer ago we consider, the less mobile people were and the smaller the population, so there are sure to be many instances of couples who share at least one pair of great-great grandparents (even this might not raise many eyebrows today).

Of course, the tree also has lots of dead ends. War, diseases, calamity, genocide, and predation were all much more common ways to die a long time ago, and often would strike before genes were passed on.