For me, in direct response to the title:
All life comes into its own as free and equal in dignity, rights and consideration. Such life as is endowed with reason and conscience also must observe this truth in transactions with any other form of life.
As an individual that also places value on non-human life, the restriction to human beings seems relatively arbitrary. Moreover, the usage of brotherhood comes off as a bit chauvinistic. All equal in dignity, we're just also going to ignore the existence of women and non-binary transgender identifying individuals in the next sentence. Starting with these two targets, I worked to modify the statement.
My personal adaptation would shift from usage of human beings to life. Fortunately, for extension purposes, life isn't defined exceptionally well right now, but definitely includes humans. Unfortunately, that opens up what should be a relatively straightforward statement to a certain degree of discussion. Oh well. That's why there's a whole stack exchange to discuss this issue.
Likewise, I append consideration to dignity and rights. This has the purpose, in human systems, of also mandating treatment in excess of the minimum required, and in non-human systems, of acting towards other life in accordance with its specific conditions and needs. That is to say, when I take consideration of other humans, I give of my time and resources to others. When I take consideration of other life, I choose vegan dietary options such as do not harm others and respect eco-systems even at non-zero financial cost.
Moreover, I can conceive of both Homo sapiens that may or may not be captured by the term "human beings" that lack reason or conscience and, for those reasons, should not be held to the same standards as other human beings. Likewise, with the extension beyond human rights, restricting reason and conscience to humans in particular becomes meaningless. For that reason, I developed a special class characterized exclusively by capacity for reason and conscience, and charged it with the implementation of my statement (as no other life would be capable of reading, understanding, and acting on it).
I'm left with something that isn't really a Universal Declaration of Human Rights, but, in my opinion, largely subsumes the Declaration, and also address broader and more difficulty issues with its provision for just treatment of non-human life.
==================================================================================
As it stands today, I highly doubt the declaration would be applied to any non-human life, even that with reason and conscience. Historically, xenophobia always supersedes tolerance, especially in first contact. Without specific and intentional provision for rights of non-human life, I would expect no rights to be afforded. However, human is an open concept. As the Declaration refrained from usage of scientific terminology, currently we are working with (thanks Google!) the following definition:
a human being, especially a person as distinguished from an animal or
(in science fiction) an alien.
Now, that's pretty imprecise. Humans are animals, and there's only a differentiation from aliens in science fiction. You'd probably lose the debate, but it is possible to argue that the "human" term could include any form of life with reason and conscience (in fact, you could even argue so on basis on the Declaration). If we encountered non-human sentient life tomorrow, I would argue for its protection on this basis.
==================================================================================
Lost human tribe? I'm more optimistic about this. People love things like themselves, and we'd all at least be within the same Genus (right, we're in the same Genus at least?). Especially as human and Homo sound somewhat similar, I'm sure many would be fine calling them humans. I hope that's reasonable.
In terms of actual ethics, well, you see what I think about respect in my generalization. I believe we would absolutely be bound to treat them (as anything else) with the utmost respect. In accordance with the Declaration, I believe we would as well. However, these are not ironclad truths descending directly from the wording of the declaration, so your mileage may vary.
==================================================================================
I think if we formulate the question as "Oh I can treat x in any way I want with no moral implications, right?" we have a fairly straightforward philosophical answer. The Declaration is a subset of this answer when x is taken to be consensus reality's definition of humans. That reduces this to a question of consensus reality, which starts to drift into theoretical sociology, so I'm fairly comfortable taking the question this far and no further.
