There isn't a single gene that definitively proves a common ancestor between yeasts and humans. Instead, it's a combination of shared genetic features, like the presence of specific genes and the organization of their genomes, that points to a common ancestor.

Here's why:

* Universal genes: Many genes, like those responsible for basic cellular functions (e.g., DNA replication, protein synthesis), are found in both yeasts and humans. This suggests they were present in a very ancient ancestor that gave rise to both lineages.

* Homologous genes: Some genes in yeasts and humans are very similar in sequence and function. These homologous genes indicate a shared evolutionary history.

* Genome organization: While yeast and human genomes are drastically different in size and complexity, the arrangement of some genes and genetic elements, like the presence of introns (non-coding sequences within genes), show similarities suggesting a common ancestor.

The key takeaway is: The evidence for a common ancestor comes from the accumulation of many shared genetic features across a vast number of genes, rather than a single specific gene.

Here's a specific example:

The gene Hsp90, which is involved in protein folding, is present in both yeasts and humans. While the sequences of this gene differ, the functional similarity points to a common ancestor.

Remember, the study of evolution relies on a vast body of evidence, and genetics is just one part of the puzzle. Studying fossils, developmental biology, and other fields all contribute to our understanding of shared ancestry.