In what sense do the Yahoos represent present humanity?
This makes the assumption that that Yahoos do represent present humanity. I don't think that is true. Though certain members of humanity may share Yahoo like tendencies, as a whole, humans are not really like the Yahoos.
Here are the similarities and differences, as I see them:
They do look similar to humans, but not "present" humans (maybe aside from my uncle Joe):
"Their shape was very singular, and deformed...Their heads and breasts were covered with a thick hair, some frizzled and others lank; they had beards like goats, and a long ridge of hair down their backs and the fore parts of their legs and feet...They climbed high trees as nimbly as a squirrel, for they had strong extended claws before and behind, terminating in sharp points, and hooked."
So they sound more like monstrous cave-men than present humanity, though they share most characteristics.
"When the beast felt the smart he drew back and roared so loud that a herd of at least forty came flocking about me from the next field, howling and making odious faces."
This shows some elements of humanity as we know it...the Yahoos seem to be social animals and work together in some ways (in this case, for defense.) Still, they are more like Gorillas than men.
"In the midst of this distress I observed them all to run away on a sudden as fast as they could,"
They are easily startled and frightened by more powerful foes. I suppose people are like that today.
The Yahoos are brutal and primitive, used as labor by the horses and treated as beasts of burden. I don't know that the author intended the Yahoos to be compared to humanity...I think the satire is more found in the horses who dominate the men.
If you wanted to make some BROAD generalizations, you could say that the Yahoos are unintelligent, savage, and driven by impulse and that humans today share some of those same qualities. But they are only shared by SOME people in the world today...there is as much decency in people as their is wickedness (though we don't always see it as such.)
Anyhow, sorry I couldn't answer your question as it was written...I just don't see the comparison except in the loosest sense. That, I think, is the answer...the Yahoos DO NOT represent modern humanity. Maybe the horses do...