You may be thinking this is old news. Didn't they announce the completion of the Human Genome Project back in 2001?
Actually the version released in 2001 was only a rough draft. Since then, scientists have been working hard to fill in some gaps and do some very important proofreading. They're still not 100% done, but they've done as much as they can with the current technology.
Each time they check over the genome sequence and get a better picture of our DNA, the number of genes they say we have goes down.
Before our DNA was sequenced, people thought we'd have 100,000 genes. Once we got the rough draft in 2001, the number fell to around 30,000. Now the final draft of the human genome puts the number between 20,000 and 25,000 genes.
Why were initial estimates so far off? And why does it matter?
Before we started to figure out our DNA, scientists looked at a number of other organisms. The simple bacterium E. coli was found to have about 4300 genes. The little worm C. elegans, which only has about 1000 cells, was found to have 19,000 genes.
So since we are so much more complex than these organisms, everyone though that our genome would hold many more genes. Looks like this isn't the case.
It's not that we are not as complicated as we thought we were. What scientists got wrong was that if something is complicated, it needs more genes.
What it looks like is going on is that we are more complex because we have other things going on besides genes...at least the way we used to think about them. We have lots of little bits of DNA that code for RNAs that never code for proteins but instead control the RNAs that do.