There are lots of different forms of mortality or immortality in Biomega:

First and foremost the so called true immortals: Reload and Vief aka Yion, origin unknown.

Then the immortal Loew, who's immortality is very closely related to the "original" as his body is designed using an artificially fertilized egg from Vief.

After that we have Narain, who used braintransplant and an artificial body to stay alive; we don´t know, however if the tentacle body or his later one were designed to be mortal or immortal, and if it was related to the original immortality.

Then we have the synthetic humans. I don´t know if THEY are in anyway related to the original immortality, but they seem to be a lesser version of them, in some aspects:
It is unclear if they just have an enormous lifespan or not, but none of them died a natural death. They have very high regenerative facilities, but those are connected with their energy levels (I think) and pale in comparison with Yions regeneration rate.

And there is Nyaldee: We know, that she managed to stay alive for quite some time, and we know that she has supernatural powers. However her origin is unknown as well.

The drones are alive, but without self consciousness. But the drone hazard was caused by the strange insect found on Mars.

Also we have the N5S manipulated humans: Again, it´s unclear what that manipulation did to their lifespan, but they seem to be generally enhanced.

How are the immortals connected to the insect?

Yes maybe, maybe, maybe...

I love coming up with crazy stuff, but i expect a little more from Nihei.
There are very obviously some strong themes going on.

If you think about it that way:
at the core we have an artist who wants to make a new manga based on the theme of immortality.

So he brainstorms a while to see, with what he comes up.

What do immortals who have all the time of the world ?
[answer: they emigrate to foreign planets]

How can they memorize such a long life ?
[answer: they dont, at first they draw like a three year old and later they just look blank most of the time]

Is it ethical or even practical for all humanity to become immortal ?
[answer: ask Nyaldee, Zouichi is to busy doing his job for mankind]

What kind of immortals/immortality do we have in mankinds mythology ?
[answer: gods, zombies, egyptian mummies (ankh)]

If you are the only immortal around dont you get lonely ? How often do you marry ?
[answer: here is a apathetic blank stare from Reload for you]

After that you begin to play around a bit, with those ideas and build a story around those themes. Preferably one with loads of action. People love that.
You must make the story possibly pretty complicated, so the readers dont get what was going on at the core, except if they spend hours on messageboards and re-read Biomega circa 3546 times and take notes at the same time.

Being unemployed may help as well.