Scientists in Italy have found that baby chickens associate low and high numbers with left and right, respectively - just like humans.
Humans are known to use a "mental number line" to think about quantities but this innate left-right association has not been seen in animals before.
The work appears in Science magazine.
Dr Rossa Rugani, who led the experiments at the University of Padova, said it was impossible to know exactly what drove the chicks' choices - but the results were clear.
"All we can judge is behavioural responses. Therefore, we don't actually know if it is a real 'number line' but it strongly resembles what is observed in the human number line," she told BBC News.
Humans show very consistent associations between spatial locations and numbers, but it is unclear how much of this develops automatically and how much we learn.
At seven months of age, babies prefer arrangements that increase from left to right. Adults show a bias whereby we respond to large numbers faster with our right hand, and smaller numbers faster with our left.
But that bias is reversed in adults who were educated in Arabic, which reads from right to left. So does education produce this bias, or just decide its direction?
Ancient origin Because we can test their behaviour very soon after hatching, chicks are "a preferred model to study the foundation of cognitive abilities" Dr Rugani explained.
She and her colleagues designed their experiments to find out whether these young creatures, only distantly related to humans, also link smaller and bigger numbers with different spatial locations.They trained each of the three-day-old animals to retrieve food from behind a sign, displaying a number of shapes.
Then the chick was offered two copies of a different number, and the researchers counted how many times each animal chose the left- or right-hand option.
If the new number was smaller than the training number, the fluffy subjects went left about 70% of the time; similarly, they went to the right 70% of the time if the new number was larger.
Just as has been shown in human studies, the effect was relative: chicks went to the right when shown "eight" if the initial "target" number was five - but when "eight" was preceded by a target of 20, they tended to go to the left.
In further experiments, the team tweaked the presentation of the numbers and found that the trend was the same even when the smaller number was presented using larger shapes, different colours, and so on.
Dr Rugani argues that her results, showing persistent left-right trends in baby birds, suggest an ancient evolutionary origin for our "mental number line".
Rather than picking up such biases as a product of our culture and education, we may have evolved these tendencies a very long time ago.
They may even have an origin in the physical arrangement of our brains, the team suggests.
Humans and animals do not have perfectly symmetrical brains and some functions are also biased to one side or the other - for example, a process called "pseudoneglect" means we pay slightly more attention to objects in the left side of space than the right. This has been proposed as an explanation for why we often start counting a series of objects with the one on the far left.
Dora Biro, an associate professor of animal behaviour at the University of Oxford, told the BBC the new results were compelling.
"It certainly seems to push back the evolutionary origins of number-space mapping... to well beyond hominids," Dr Biro said.
She added that it will be interesting to see whether the left-right association can be re-trained in the same way that it can in people - or whether the animals' brains are more hard-wired to map numbers onto space in this particular orientation.