The following creationist argument is not one used by my (now ex-) coworker. He is more sophisticated in his debating. Still, the following rhetorical point is and has been used again and again, so here is my attempt at a comprehensive response.
Here is the point: If humans came from monkeys why are there still monkeys?
A common alternate is: If humans came from apes, why are there still apes?
Stephen Baldwin "If we are from apes, why are the apes still here?"
Here is a similar formulation (March 16, 2012):
This is where Kern began to argue that evolution - at least macroevolution - doesn't make sense because "viruses are still viruses" and other organisms are still what they have always been ... "You're talking about adaptation here," he told her.
Smith went on to talk further about viruses, while Kern sat there with a sour look on his face, coming back to tell her that "You go back and viruses are viruses ... they may have adapted ... they are still viruses."
When Kern asked where the virus fossils are, Smith responded that viruses are too small to fossilize.
The short answer is clear and simple.
Apes are not, evolutionarily speaking, our ancestors but our cousins. We both evolved from a common ancestor.
The viruses that exist today may or may not be similar to ancient ones but they too have evolved. It is possible that a virus was the first sorta-lifeform and more complex animals evolved from it, although I have not heard that claim stated anywhere. Still, viruses today are distant relatives, not ancestors.
There is a larger question and it could be stated as "If humans are so superior, why do any other animals exist?" or "If humans are so successful, why do other animals exist?"
Dawkins discussed primitive and advanced animals in The Ancestor's Tale:
...Darwin's advice to himself would serve us all: 'Never use the words higher and lower'.
Lancelets are live creatures, our exact contemporaries. They are modern animals who have had exactly the same time as we have in which to evolve. Another telltale phrase is 'a side branch, off the main line of evolution.' All living animals are side branches. No line of evolution is more 'main' than any other, except with the conceit of hindsight. (Page 365 of First Mariner Books edition, 2005, The Ancestor's Tale. This is the third or fourth paragraph from the start of the Lancelet chapter)
From the Wikipedia article on The Ancestor's Tale.
But lancelets are not primitive nor our remote ancestor. They are as modern as all other members in the pilgrimage. The Lancelet's Tale continues to develop the theme introduced in The Duckbill's Tale, that all living animals have had equal time to evolve since the first concestor, and that no living animal should be described as either lower or more primitive.
Perhaps an example from the military will help. I have no background in the military but I think this metaphor works. In the ocean, the most powerful vessel is the aircraft carrier. And yet, this does not mean that the only military vessels used today are aircraft carriers. Missile launching and countering vessels are needed as are mine sweepers and submarines. A vessel, or an animal, can be very strong and successful and yet not be supreme in all areas.
Below is the beginning of a list I started looking at successful animals. I count success as a combination of numbers and total mass. Ants are on the list as they outnumber us by so much that their total mass is similar to ours. Although a recent discussion of viruses encouraged me to write this post, I did not add microscopic animals to the list.
chickens: "More than 50 billion chickens are raised annually"
sheep: 1.078 billion
There are around 5 billion rats here on earth, which makes them arguably the second most successful mammal. After humans, of course. An average rat weighs around 450 grams. That makes for almost 5 billion pounds of rat. They are about 25 centimeters long, or 10 inches. That makes 1 and a quarter million kilometers of rat.
Blue whales are the biggest animal that has ever existed on this planet. They weigh almost 400 thousand pounds each. There are approximates 12000 blue whales on this planet. The weight of all the blue whales, is almost 200 million pounds less than the weight of all the rats.
It has been estimated that the total ant population is around 10 quadrillion, or 10,000,000,000,000,000. That’s a lot of ants. It’s more than 1.4 million ants for every person.
An average ant weighs about 20 milligrams, which is almost nothing. But that means all of them together weigh around 440,924,524,000 pounds. So do they outweigh humans? Unless the weight of an average human is only 63 pounds, then no. But let’s look at it another way.
Ants aren’t very good at being heavy. One thing they are good at though, is lifting a lot of weight. An ant can lift around 50 times its own body weight. So every ant in the world combined could lift more than 22 trillion pounds. If the human population wanted to lift that much, we’d each need to lift more than 3000 pounds. Which is more than 6 times the world record for weight lifting.
So my list, rather unimaginatively, consists of only two groups, human food items and detritivores with two examples of each. I am so ignorant of ocean ecology that I feel unable even to suggest a fish to search online for. Krill could also be on this list, I guess.
The lesson could be: there are a variety of ways of being successful. Being small and having many offspring appears to work the best. With this in mind, why are there still humans?