Johannes Gerardus Marie (Hans) Thewissen is a Dutch-American paleontologist known for his significant contributions to the field of whale evolution.

In addition to his work on fossil discoveries, Thewissen also studies modern bowhead and beluga whales in Alaska, focusing on their biology and the implications of this knowledge for management and conservation efforts.

His research has been instrumental in deepening our understanding of cetacean evolution and the adaptations that allowed these mammals to transition from terrestrial to fully aquatic lifestyles.

His mother said that when Thewissen was a small boy, she had to sort through his pockets before laundry time to take out the rocks and worms he collected.

[2] After finishing Gymnasium secondary education in Deurne, he completed undergraduate degrees in biology with a minor in geology in 1981 at the University of Utrecht.

He studied for a semester at the Museum National d'Histoire Naturelle in Paris, before moving to the U.S. to earn a PhD in Geological Sciences at the University of Michigan, where he studied phenacodontids, a group of Paleogene ungulate mammals (condylarths) that is ubiquitous in North America (more than 6,000 fossils), but rare or absent in all other continents.

[12][13] "I sat on the porch of a Pakistani guesthouse, puzzling over the sea lion-size skeleton that we had just dug up in the Kala Chitta Hills.

I opened some of the packages containing fossil remains that I had wrapped earlier that day, and as I scraped with a dental tool, I realized that this was a whale—one that could walk around on the large hind legs that we had unearthed.

"[14][15] Sunil Bajpai and Thewissen collected fossils in District Kutch, State of Gujarat, India, in the desert area close to the Pakistani border.

The holotype of Kutchicetus consists of some skull fragments, many vertebrae, and ribs and the limb bones, although parts of fore- and hind feet were not found.

In the cracked specimen, Thewissen recognized the ear structure of the auditory bulla which had a shape which is highly distinctive, found only in the skulls of living and extinct cetaceans, including Pakicetus.

[20] Thewissen was able to extract many skeletal bones of Indohyus, which showed that the species was similar in body shape to a modern mousedeer (also called chevrotains).

[23] Fred Spoor, an anthropologist at University College London, said the significance of the latest find was comparable to Archaeopteryx, the first fossils to show a clear transition between dinosaurs and birds.

To gain access to modern whale soft tissues, Thewissen began traveling to Alaska's north slope.

Working in collaboration with the Department of Wildlife Management of the North Slope Borough, Thewissen gained access to the small number of bowhead and beluga whales which are captured yearly by native Alaskans as an indigenous exemption to the Marine Mammal Protection Act.

Suzana Herculano-Houzel has developed a method of counting of neurons of human and other animals' brains and the relation between the cerebral cortex area and thickness and number of cortical folds.