Johann Karl Ernst Dieffenbach is said to have been born at Giessen in the Grand Duchy of Hesse on 27 January 1811. The son of a respected Protestant cleric and professor of theology at the university of Giessen, Ernst enrolled in the faculty of medicine in 1828 and was caught up in the excitement of the rapidly expanding realm of physical science. He also joined the agitation for political reform and national unification, became a political fugitive in Switzerland, and in 1836 was imprisoned in Zurich for two months, plus one week for duelling. But he had not neglected his studies. Before being expelled from Switzerland in 1836 for political activities, he was accepted for the degree of doctor of medicine at Zurich. He reached England via France in 1837.
In London, Ernest Dieffenbach, as he became known in England, soon began to make his way in scientific circles. He knew Charles Darwin and Charles Lyell and later came to regard Richard Owen as a friend and mentor. Owen, who in 1837 deduced from a fragment of femur the existence of a large flightless bird in New Zealand, and Darwin may have spurred Dieffenbach's interest in the position of naturalist with the New Zealand Company. He was nominated by two officials of the Royal Geographical Society and sailed aboard the Tory in May 1839. William Wakefield, Edward Jerningham Wakefield and Charles Heaphy were fellow passengers and would have been lively company; Dieffenbach's choice of subject for their debating society – 'The causes of the decay of Nations and whether it be possible to prevent the decay of a Nation' – suggests a compensating solemnity.
Dieffenbach made important journeys in the Marlborough Sounds, the Hutt Valley, Taranaki, along the west coast of the North Island into the central volcanic regions, and in Northland, and spent about four weeks at the Chatham Islands. With James Heberley he reached the summit of what he percipiently called 'Taranaki, or Mount Egmont', on Christmas Day 1839; this is generally reckoned to be the first ascent by a European.
Dieffenbach was by no means the New Zealand Company's unquestioning servant, soon concluding that the Wakefield land purchase scheme would lead to speculation, while he saw Wellington as an unpromising site for settlement. He later quarrelled with the company about the value of his collections and reports. He did not like what he saw of settler society, 'the imported race of shopkeepers…who pride themselves on their ignorance regarding everything that belongs to the native inhabitants', and vehemently denounced the CMS missionaries for their land purchases. Yet he gave good value, reporting fully to the company on his observations and methodically measuring tides, temperatures, heights and distances, as a guide for the newcomers.
Dieffenbach was the first trained scientist to live and work in New Zealand. His collections from the flora and fauna found their way eventually to the Royal Botanic Gardens, Kew, and the British Museum; he is said to have collected 58 species of shells. The Chatham Island rail, Rallus dieffenbachii, and some species of fishes and plants perpetuate his name. He seems to have been the first to apply the term 'greywacke' to the ubiquitous hard sandstone of the New Zealand ranges, and his observations about the North Island volcanoes and related phenomena were original and pertinent. Perhaps he was more a man for the quick run over the ground than for investigative research. Even so, he might have made a greater mark if he had succeeded with his petition in 1841 to stay on to complete a scientific survey of the country after his contract with the company ran out. He persuaded Lieutenant Governor William Hobson of the need. Higher officialdom in Sydney and London, however, turned the proposal down flat and Hobson, that luckless administrator, was read a lecture in economic management of the colony.
Dieffenbach returned to England in October 1841. His book, Travels in New Zealand, published in London in 1843, broke new ground in joining a meticulous account of the life and physical character of the new country with penetrating and humane observations about the plight of the Māori before the rising tide of European settlement. Dieffenbach looked at New Zealand with the fresh and sensitive eye of a nineteenth century liberal. He picked the importance of land and language to the survival of the Māori people and made a strong plea for special measures to protect what he called 'a magnificent race, people of a fine and gentle disposition'.
Dieffenbach's imagination was taken not by the land, but by its people. His writings on Māori life include a serious study of the language, and are infused with compassion, curiosity and respect. He was despondent about the creeping decay of Māori health, character and institutions as European influence spread. He laid out some sensible administrative prescriptions for the preservation of Māori interests. With remarkable insight he identified the 'ruling spirit of English colonization…absolute individuality' as at odds with the communal lifestyle and culture of the Māori. At the heart of it all was the deep spiritual attachment of the Māori to tribal lands. That must be recognised and tenure guaranteed.
With such beliefs, strongly held, Dieffenbach could have been a powerful influence in the developing New Zealand society. As it was he had no option but to return to London, where he resumed a precarious career as a translator and scientific jack-of-all-trades. He had an interview with Lord Stanley with the aim of returning to New Zealand in 1846, but again was unsuccessful. He was able to visit Giessen but not to live there until 1848, when the political slate on the events of the early 1830s seems to have been wiped clean. Dieffenbach was nominated as a deputy in the short-lived national assembly formed during the 1848 revolution. He declined and instead took up an appointment as associate professor in geology at his old university, becoming director of the geological museum. Dieffenbach married Katharina Emilie Reuning in April 1851. They had two daughters, Klara and Anna. He died of typhus in 1855, probably on 1 October.