We’ve all heard the wisdom that one human year is equivalent to seven dog years, but newer research is rewriting the formula for human-to-dog age comparisons. The “multiply by 7” method comes from dividing the human lifespan, of about 80 years, by the average dog lifespan of 12 years, but it appears the math is more complicated than that.

According to a study published last summer in the journal Cell Systems, the DNA of our canine companions appears to give off a similar pattern of signals to mark periods of development as what is observed in the human aging process. For example, a comparable biological signature was observed in an eight-week-old puppy as what is observed in a nine-month-old baby, corresponding to the period of infancy in both dogs and humans. The researchers examined the epigenome, or the series of surface modifications to our DNA that turn genes “on” and “off,” to better understand how biology dictates when life stages happen. The study — from researchers at the National Institutes of Health (NIH), University of California system, and the University of Pittsburgh School of Medicine — adds to an already compelling case as to why dogs might be the key to understanding human aging.

Veterinarians have long known that our canine companions are useful models of human health. After all, humans and their pet dogs live in the same environments, eat similar foods, and are exposed to the same pollutants and chemicals. Many of the diseases and conditions that afflict humans are also found in dogs, except they unfold in a more compressed period of time due to dogs’ shorter lifespans. These shared characteristics make the domestic dog an instrumental model for conditions of human health.

Developmentally, humans and dogs age in similar ways, too; we each have life stages of birth, infancy, youth, puberty, adulthood, and death, and we share the same biological markers of aging.

This latest study homed in on those biological markers, specifically those found in the epigenome. The epigenome represents all of the chemical changes that happen “on” or “above” the genome, or mechanisms that affect the surface of DNA rather than the sequence of it. These include histone modifications that change how DNA is packaged, or DNA methylation that adds methyl groups (CH3) to a DNA molecule — both of which work to silence or express particular genes.

Patterns of DNA methylation, in particular, are known to change as humans age in a predictable manner, so much so that scientists can predict the age of an individual just by looking at their sum total of DNA methyl groups — i.e., their methylome — at a snapshot in time. These predictions are made using mathematical models called epigenetic clocks, which, until this study, have not been able to accurately predict age in other species, only in humans. This limitation is due in part to technology; while scientists know how to observe the methylome in a human, there is no method available to study the methylome in other mammals.

Thus, leveraging the known similarities between humans and dogs, the researchers set out first to develop a method to capture the methylome in a dog, and secondly, to compare methylation patterns between dogs and humans to better understand the dog-to-human age comparison.

The team took blood samples from 104 Labradors, aged four weeks-old to 16 years-old, and compared their methylation patterns to a published dataset of methylation patterns from 320 people, aged 1 to 103 years-old.

The results found similar age-related methylation patterns between young dogs and young humans, as well as more senior dogs and older humans. However, the opposite was not observed: young dogs do not have the same methylome as an older human, and vice versa. In the study, an eight-week-old puppy was roughly the same age as a nine-month-old baby, and 12 years in a Labrador was equivalent to about 70 years in humans.

The researchers also discovered that specific groups of genes involved in development had similar methylation patterns in both dogs and humans, meaning future studies might be able to target particular genes to see how they affect the aging process.

Future work will also attempt to expand the dog-to-humans formula using other dog breeds to see if the epigenetic clock devised in this study is also accurate for breeds other than Labrador retrievers.

Elaine Ostrander, a co-author on the study and the chief of the Cancer Genetics and Comparative Genomics Branch at the National Human Genome Research Institute of NIH, said in this NIH press release that the study “highlights the relevance of canine aging studies,” and further positions the dog as a model system to “inform human health and biology.”