A science project has generated a new, higher-quality assemblies of great ape genomes, without the guidance of the human reference genome.

The findings published on Thursday in the journal Science helped reduce "humanizing" discovery bias in genomes of orangutan and chimpanzee, providing a clearer view of the genetic differences that arose as humans diverged from other primates.

The researchers led by Zev N. Kronenberg, a computational biologist, and Evan Eichler, a geneticist, both of them from the University of Washington, sequenced more than 500,000 full-length genes from each species.

The newest investigation provided the most comprehensive catalog of genetic variants that were gained or lost on different ape lineages.

Some of these variants affect how genes are differentially expressed among humans and apes, according to the study.

The researchers examined the possible influence of some of the genetic variants and gene function regulators on such areas as human and ape dietary differences, anatomy, and brain formation.

For instance, the researchers studied brain organoids which were laboratory-grown tissues coaxed from stem cells of apes or humans and forming a simplified version of organ parts.

These brain proxies were examined to try to understand how differences in gene expression during brain development in humans and chimps might account for chimps' smaller brain volume, which is three times less than human brain volume.

The researchers observed in the organoids that certain genes, particularly those in cells that are like the progenitors of radial glial neurons, are down-regulated in humans compared to chimps.

Those genes were more likely to have lost segments of DNA specifically in the human branch important in regulating their expression, the researchers said.

They said that this finding was consistent with a "less is more" hypothesis proposed in the 1990s by genome sciences professor Maynard Olson and his colleagues who proposed that the loss of functional elements contributes to critical aspects of human evolution.

Humans also underwent the deletion of some genes involved in the synthesis of fatty acids. Some genetic changes related to dietary metabolism were identified in this project. These may have played a role relevant to the evolution of ape species. Great ape diets range from keeping strictly vegetarian to eating almost anything.

On the other hand, certain human genes appear to be linked to up-regulation for neural progenitors and excitatory neurons in the nervous system. These genes are more likely to have gained additional copies in the human species, compared to other apes, through a process of gene duplication.

The researchers said that their recent findings fit with their previous studies showing that the genomes of the common ancestral lineage for African great apes likely underwent an expansion of segmental duplication more than 10 million years ago.

These repeats of sections of the genetic code may have made great ape genomes particularly prone to deletion and duplication events, thereby accelerating the rate of mutations with major consequences that helped drive the evolution of ape species.

"Our goal is to generate multiple ape genomes with as high quality as the human genome. Only then will we be able to truly understand the genetic differences that make us uniquely human," said Eichler.