When it comes to the natural world and today's advocates for it, Jane Goodall is one of the first names that comes to mind.
The primatologist and renowned environmental activist is most famous for her work with chimpanzees, living alongside the species for several years in Gombe, Tanzania when she was in her early 20s.
Goodall's work led to a number of important discoveries, including that these animals could make and use tools, and feel and express emotion in a similar way to humans.
At 83 years old, Goodall has seen a lot of the world - she still travels about 300 days a year.
Despite the sometimes challenging issues that face our environment today, she is still positive and hopeful about the future, something she will be discussing on her tour of New Zealand which started in Dunedin last night.
"I have to think positively about the future of our planet because people tell me after my talks that they have changed their lifestyle, and people genuinely do care," Goodall says. "And I've seen change."
"We're not winning the battle. Not even close, but there are a lot of things I see that still make me hopeful."
Goodall's main message to the world, and one she will be focusing on at her talks in Dunedin, Wellington, Christchurch and Auckland, is one she has been spreading for years - It's not too late for the planet and its inhabitants as long as we take action and start to make changes.
"The big issue is that as the problems get worse and the attacks on the environment get worse, ordinary people just feel there's nothing they can do and they lose hope and do nothing, and that's the disaster," Goodall says.
"But more and more people are beginning to think about the choices they make and to see that all of these choices are important. It isn't too late."
As Goodall puts it, her five main reasons for hope are simple. Firstly, the human brain and the new innovations being created every day are limiting the destructive impact humans are having on the environment.
Secondly, the resilience of nature, which she highlights using the example of New Zealand's Black Robin. At one point the species had just one fertile female left.
Today there are several hundred of the birds and although still endangered, the population managed to survive near extinction.
Thirdly is what Goodall describes as the indomitable human spirit and the passion of individuals in trying to make the world a better place.
Fourthly and most recently is the explosion and power of social media, with people much more aware and able to participate in events and organisations and to be more connected than ever before.
And finally, the energy and commitment of today's youth - something Goodall is very involved in through setting up the Jane Goodall Institute's Roots and Shoots programme.
New Zealand is one of more than 100 other countries involved in the programme, in which young people from kindergarten through to university select three projects to work on - one to help people, one to help animals and one to help the environment.
"Humans can do the most amazing things. We can send rockets into space and build incredible things, and yet we are destroying our home," Goodall says.
"But at the end of the day, I think we can still turn things around. Everywhere I go people tell me what they are doing to make a difference and that makes me hopeful," she says. "And we have to be hopeful."
An Evening with Dr Jane Goodall - Dunedin June 25, Wellington June 26, Christchurch June 29, Auckland July 1.