When William Erasmus Darwin was born in December 1839, his father Charles began to meticulously record observations of his firstborn in a notebook.

Now housed at Cambridge University Library, it reads more like a research document than like that of a new parent blissfully observing his son's behaviour, as the opening comments reveal:

"During first week, yawned, streatched [sic] himself just like old person – chiefly upper extremities – hiccupped – sneezes sucked…."

Today we know a good deal about Darwin's theories. We know far less about how his private life – particularly his family – contributed to his work. But his vast collection of letters and the notebook reveal an intriguing side to the founding father of evolution: Darwin as a family man.

What's more, his children's development helped inform his understanding of human evolution.

Darwin's son William was born a year after the scientist first met London Zoo's first orangutan, Jenny.

"The orangutan for Darwin was like a window into the origins of mankind," says John van Wyhe of the National University of Singapore and director of Darwin Online, which hosts a collection of all of Darwin's published works.

Many parents make notes about their children but not quite like this

At this time, Darwin was already forming ideas about where humans came from, but he had never met one of our close ape relatives to test these theories. His encounter with Jenny helped cement his idea that that we share a common ancestor with apes.

He was already looking for a "real relationship between humans and apes", says van Wyhe. When he saw Jenny's facial expressions and noticed her social behaviour, it reaffirmed his ideas.

What stood out to him was how human-like some of her behaviour was, which he wrote about in a letter to his sister Susan:

"The keeper showed her an apple, but would not give it her, whereupon she threw herself on her back, kicked & cried, precisely like a naughty child."

When his son was born soon after, it meant that he could see first-hand how a human child developed, and consider the relationship between humans and animals. At times, he even referred to his son as "it".

I made loud snoring noise, near his face, which made him look grave & afraid & then suddenly burst out crying

Many parents make notes about their children but not quite like this, says Alison Pearn, of the Darwin Correspondence Project at Cambridge University Library. "This is very much a set of research notes."

The notebook provides an intimate glimpse of Darwin as a good-humoured though curious father who is "prodding and poking his young infant like he's another ape," says van Wyhe.

His initial observations about both his family and Jenny the ape went on to influence his 1871 book The Descent of Man and The Expression of the Emotions in Man and Animals, published in 1872.

In his notes on William, Darwin experimented with noises that made his son afraid:

At times, Darwin even referred to his son as 'it'

"I made loud snoring noise, near his face, which made him look grave & afraid & then suddenly burst out crying... I repeated the experiment."

In his Emotions book he later wrote about how humans first react to noise. He tested his ideas on his children by rattling a box in front of them. "My infants started at sudden sounds, when under a fortnight old..." Apes, he found, also readily expressed fear: "An orang, also, has been known to be much alarmed at the first sight of a turtle."

Darwin also considered happier emotions. He noted when William first learned to smile: at five weeks his smile was only a "chance movement" but at six weeks he smiled with his eyes.

Apes, he wrote, smile in a similar way: "their eyes sparkle and grow brighter". When young orangutans smile they resemble an expression "often seen on the face of man".

We have been able to read many of these observations ever since they were published, but in 2015 there was another surprising discovery. Hidden away in Cambridge University Library, van Wyhe and Peter Kjærgaard of the Natural History Museum in Denmark uncovered Darwin's initial observations.

His notes were found on two sheets of paper fittingly titled "Man" – despite including barely a mention of humans (though Jenny was dressed in human clothes to amuse visitors).

Again they reveal a stark recollection of emotions once considered to be uniquely human. For one, Jenny was "decidedly jealous" when given insufficient attention. She also "covered itself herself up with two pocket handkerchiefs just like girl with shawl spread them out."

Three or four days ago smiled at himself in glass – how does he know his reflection is that of human being?

Orangutans "are curious, particularly fond of watching boys bathe" and "like a child when annoyed", he wrote.

He was also beginning to grasp whether orangutans were self-aware, which we see today as a hallmark of consciousness. Scientists can test this using the simple "mirror test" to see if an animal can recognise itself in a mirror.

In 1838 Darwin unwittingly stumbled upon an early version of this test (which was only officially developed in 1970). He saw that Jenny and another orangutan "made ugly faces (especially at the glass)… Both were astonished beyond measure at looking glass, looked at it every way, sideways, & with most steady surprise."

Intrigued, he later went back to understand more about how orangutans behaved in front of a mirror, and would later ask similar questions about his son:

"Three or four days ago smiled at himself in glass – how does he know his reflection is that of human being? That He smiles with this idea, I feel pretty sure."

Fortunately for Darwin, his wife Emma knew what she was signing up for, including becoming a possible subject of his scientific observations, before they married.

Darwin's daughter Henrietta arguably played an even more important role: that of his editor

"I believe from your account of your own mind that you will only consider me as a specimen of the genus… You will be forming theories about me & if I am cross or out of temper you will only consider 'What does that prove'", she wrote in a letter to Darwin in January 1839.

Later, Darwin's children would go from being scientific subjects to his helpers.

Cambridge University Library recently acquired a deposit of 112 previously unknown letters from two of his children, Henrietta and William. With his other correspondence, they reveal how involved his children were in his studies.

In one letter about an evening ball, William tells his father his observation of a fly sucking a plant. "I don't know whether Etty [Henrietta] told you of my seeing a fly? Sucking orchis maculata," he wrote.

His family also seemed to have a great deal of fun together

"We had thought of him as being slightly detached," Pearn says of William. "The letters show that he sent his father observations and discussed ideas."

Darwin's daughter Henrietta arguably played an even more important role: that of his editor. Darwin even bothered her while she was on holiday in Europe, asking her to read through his Descent of Man manuscript.

"After reading once right through, the more time you can give up for deep criticism or corrections of style, the more grateful I shall be," he wrote.

Fortunately, it was not all work in the Darwin household. In fact, Darwin was never paid as a scientist. Family wealth meant his work as a naturalist was voluntary: though he worked his whole life, he never had to.

You can just imagine him raiding the kitchen to feed the plants

His family also seemed to have a great deal of fun together, says Pearn. "It's really an enviable family life. The science was everywhere. Darwin just used anything that came to hand, all the way from his children right through to anything in his household, the plants in the kitchen garden."

The family raised cockerels, pigeons and rabbits. Exotic and experimental plants littered his parlour windowsill and pantry.

Some of these were carnivorous. Darwin experimented by feeding them all sorts of foods. "Hard-boiled eggs, bits of cooked meat and olive oil – so you can just imagine him raiding the kitchen to feed the plants," Pearn says.

Darwin's ideas still permeate science today. Perhaps the sheer number of his ground-breaking ideas is less surprising when you consider that he spent every waking minute, even the most intimate, observing the world around him.

"He put down his own success to close observation, an open mind and curiosity," says Pearn.

Although Darwin began publishing his ideas over 150 years ago, his early observations were eerily accurate, says Peter Kjærgaard. That is despite the absence of comparative fossils of other early humans.

All Darwin had to go on were our closest living relatives, the apes. They revealed then – as they do now – that the gap between humans and animals is slowly closing, as we have covered before.

After all, as Darwin later wrote, we differ in degree, not kind, from non-human apes.

These early observations helped start what has now become an entire field: primatologists look at our closest relatives to understand more about humanity.

In some ways, we have not only Jenny the orangutan to thank for that – but even Darwin's own children.

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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Some of Darwin's letters are now on display at Lines of Thought: Celebrating 600 years of Cambridge University Library.