LOS ANGELES — Eric Harris Davidson, a developmental biologist whose work revealed how complex organisms arise from the meeting of egg and sperm, has died at 78.

Davidson, who was Norman Chandler professor of cell biology at the California Institute of Technology, died Sept. 1 after a heart attack, said his daughter, Elsa Davidson.

In 1969 and 1971, Davidson co-authored papers that sketched, for the first time, the process by which networks of genes interact to drive embryonic growth, the proliferation of specialized cells, and the creation of organisms of vastly diverse shapes and sizes.

Two decades before the Human Genome Project would be launched, Davidson and his longtime collaborator, molecular biophysicist Roy Britten, proposed models of gene regulation that would presage much later work on the epigenome – the system of molecular marks that turn genes on and off and regulate their production of proteins to build different types of tissue.

The two concluded that networks of interacting genes define how an animal’s body plan will take shape, emerging from a pair of undifferentiated cells. And they suggested that small changes in the function of these regulatory networks could be a key engine of evolutionary change.

Their work became a cornerstone of the field of evolutionary developmental biology.

“He was a revolutionary,” said Ellen V. Rothenberg, a Caltech stem cell biologist who was Davidson’s companion and scientific collaborator for more than two decades.

“In the 1950s and ’60s, this was a fusty, stodgy field” in which scientists collected, categorized and described organisms, Rothenberg said.

“He came in and dragged them into the genomic age,” she said.

Drawing on his familiarity with “every kind of obscure invertebrate,” Davidson was determined to understand how different patterns and interactions of DNA – a molecule common to virtually all forms of life – could produce such an astonishing diversity of creatures, Rothenberg said.

“He was a cathedral builder,” she said.

He was an ardent animal lover, and had to be persuaded not to feed the black bears that occasionally wandered into the yard of his home in Pasadena’s Kinneloa Canyon.